Shift Work and Sleep Disturbances

  • T. Åkerstedt


Formally, the term “shift work” refers specifically to an arrangement of work hours that uses two or more teams (shifts) to cover the time needed for production. Whereas two-shift work usually covers only daylight hours, three-shift work also covers the night. Shifts are often changed around 0600, 1400, and (if a night shift is included) 2200 hours, although many companies employ earlier or later times. In Europe the teams usually rotate between the shifts, whereas in the United States assignment to a certain shift is often permanent, at least for a considerable time, before seniority allows transfer to another shift. Permanent night work, the watch system at sea, and roster work are other varieties of work hour arrangements. The latter (roster work) is similar to conventional shiftwork, only somewhat more irregular and “customized” to particular needs, usually in the service sector.


Circadian Rhythm Sleep Disturbance Night Shift Sleep Loss Multiple Sleep Latency Test 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. 1.
    Maurice M (1975) Shift work. ILOGoogle Scholar
  2. 2.
    Graf O, Pirtkien R, Rutenfranz J, Ulich E (1958) Nervöse Belastung im Betrieb. I. Nachtarbeit und nervose Belastung. Westdeutscher VerlagGoogle Scholar
  3. 3.
    Thiis-Evensen E (1958) Shift work and health. Ind Med Surg 27: 493–497PubMedGoogle Scholar
  4. 4.
    Menzel W (1962) Menschliche Tag-Nacht-Rhythmik und Schichtarbeit. Schwabe, BaselGoogle Scholar
  5. 5.
    Aanonsen A (1964) Shift work and health. Universitetsforlaget, OsloGoogle Scholar
  6. 6.
    Anderson FE (1970) Three-shift work. Socialforskningsinstitutet. CopenhagenGoogle Scholar
  7. 7.
    Åkerstedt T, Torsvall L (1988) Shift work and transient insomnia. In: Home J (ed) Sleep 1988Google Scholar
  8. 8.
    Ehrenstein W, Müller-Limmroth W, Schaffler K, Thebaud C (1970) Polygraphische Schlafuntersuchungen des Tagschlafs nach einer Nachtschicht und des Nachtschlafs nach einer Tagschicht an acht Krankenschwestern. Pflügers Arch 319: 121Google Scholar
  9. 9.
    Foret J, Benoit O (1974) Structure du sommeil chez des travailleurs à horaires alternants. Electroencephalogr clin Neurophysiol 37: 337–374CrossRefGoogle Scholar
  10. 10.
    Matsumoto K (1978) Sleep patterns in hospital night nurse due to shift work: an EEG study. Waking and Sleeping 2: 169–173Google Scholar
  11. 11.
    Dahlgren K (1981) Adjustment of circadian rhythms and EEG sleep functions to day and night sleep among permanent night workers and rotating shift workers. Psychophysiology 18: 381–391PubMedCrossRefGoogle Scholar
  12. 12.
    Foret J, Lantin G (1972) The sleep of train drivers: an example of the effects of irregular work schedules on sleep. In: Colquhoun WP (ed) Aspects of human efficiency. Diurnal rhythm and loss of sleep. The English Universities Press Ltd, London, pp 273–281Google Scholar
  13. 13.
    Torsvall L, Åkerstedt T, Gillberg M (1981) Age, sleep and irregular work hours: a field study with EEG recording, catecholamine excretion, and self-ratings. Scand J Work Environ Health 7: 196–203PubMedCrossRefGoogle Scholar
  14. 14.
    Torsvall L, Åkerstedt T, Gillander K, Knutsson A (1989) Sleep on the night shift: 24-hour EEG monitoring of spontaneous sleep/wake behavior. Psychophysiology 26 (3): 352–358PubMedCrossRefGoogle Scholar
  15. 15.
    Tilley AJ, Wilkinson RT, Drud M (1981) Night and day shifts compared in terms of the quality and quantity of sleep recorded in the home and performance measured at work: a pilot study. In: Reinberg A, Vieux N, Andlauer P (eds) Night and shift work. Biological and social aspects. Pergamon Press, Oxford, pp 187–196Google Scholar
  16. 16.
    Foret J, Benoit O (1978) Shiftwork: the level of adjustment to schdule reversal assessed by a sleep study. Waking and Sleeping 2: 107–112Google Scholar
  17. 17.
    Lille F (1967) Le sommeil de jour d’un groupe de travailleurs de nuit. 30: 85–97Google Scholar
  18. 18.
    Kripke DF, Cook B, Lewis OF (1971) Sleep of night workers: EEG recordings. Psychophysiology 7 (3): 377–384CrossRefGoogle Scholar
  19. 19.
    Bryden G, Holdstock TL (1973) Effects of night duty on sleep patterns of nurses. Psychophysiology 10 (1): 36–42PubMedCrossRefGoogle Scholar
  20. 20.
    Tepas DI, Walsh JK, Armstrong DR (1981) Comprehensive study of the sleep of shift workers. In: Johnson LC, Tepas DI, Colquhoun WP, Colligan MJ (eds) Biological rhythms, sleep and shift work. SP Medical & Scientific Books, New York, pp 347–356Google Scholar
  21. 21.
    Weitzman ED, Kripke DF, Goldmacher D, McGregor P, Nogeire C (1970) Acute reversal of the sleep-waking cycle in man. Arch Neurol 22: 483–489PubMedGoogle Scholar
  22. 22.
    Berger RJ, Walker JM, Scott TD, Magnuson LJ, Pollack SL (1971) Diurnal and nocturnal sleep stage patterns following sleep deprivation. Psychon Sci 23 (4): 273–275Google Scholar
  23. 23.
    Knauth P, Rutenfranz J (1972) Untersuchungen zum Problem des Schlafverhaltens bei experimenteller Schichtarbeit. Int Arch Arbeitsmed 30: 1–22PubMedCrossRefGoogle Scholar
  24. 24.
    Knauth P, Landau K, Dröge C, Schwitteck M, Widynski M, Rutenfranz J (1980) Duration of sleep depending on the type of shift work. Int Arch Occup Environ Health 46: 167–177PubMedCrossRefGoogle Scholar
  25. 25.
    Hume KI (1980) Sleep adaptation after phase shifts of the sleep-wakefulness rhythm in man. Sleep 2 (4): 417–435PubMedGoogle Scholar
  26. 26.
    Webb WB, Agnew HW (1978) Effects of rapidly rotating shifts on sleep patterns and sleep structure. Aviat Space Environ Med 49 (2): 384–389PubMedGoogle Scholar
  27. 27.
    Foret J, Bensimon B, Benoit O, Vieux N (1981) Quality of sleep as a function of age and shift work. In: Reinberg A, Vieux N, Andlauer P (eds) Night and shift work: biological and social aspects. Pergamon Press, OxfordGoogle Scholar
  28. 28.
    Åkerstedt T, Torsvall L (1981) Age, sleep, and adjustments to shift work. In: Koella W (ed) Sleep 1980. Karger, Basel, pp 190–194Google Scholar
  29. 29.
    Feinberg I, Fein G, Floyd TC (1980) EEG patterns during and following extended sleep in young adults. Electroencephalogr Clin Neurophysiol 50: 467–476PubMedCrossRefGoogle Scholar
  30. 30.
    Nakagawa Y (1980) Continuous observation of EEG patterns at night and in daytime of normal subjects under restricted conditions. I. Quiescent state when lying down. Electroenceph Clin Neurophysiol 49: 524–537PubMedCrossRefGoogle Scholar
  31. 31.
    Benoit O, Foret J, Bouard G, Merle B, Landau J, Marc ME (1980) Habitual sleep length and patterns of recovery after 24 hour and 36 hour sleep deprivation. Electroencephalogr Clin Neurophysol 50: 477–485CrossRefGoogle Scholar
  32. 32.
    Åkerstedt T, Gillberg M (1981) The circadian variation of experimentally displaced sleep. Sleep 4: 159–169PubMedGoogle Scholar
  33. 33.
    Akerstedt T, Gillberg M (1986) A dose-response study of sleep loss and spontaneous sleep termination. Psychophysiology 23: 293–297PubMedCrossRefGoogle Scholar
  34. 34.
    Wyatt S, Mariott R (1953) Night and shift work changes. Br J Ind Med 10: 164–177PubMedGoogle Scholar
  35. 35.
    Dirken JM (1966) Industrial shift work: decrease in well-being and specific effects. Ergonomics 9: 115–124PubMedCrossRefGoogle Scholar
  36. 36.
    Mott PE, Mann FC, McLoughlin Q, Warwick DP (1965) Shift work - the social psychological consequences. Ann Arbor, University of Michigan PressGoogle Scholar
  37. 37.
    Åkerstedt T, Torsvall L (1978) Experimental changes in shift schedules - their effects on well-being. Ergonomics 21: 849–856PubMedCrossRefGoogle Scholar
  38. 38.
    Verhaegen P, Maasen A, Meers A (1981) Health problems in shift workers. In: Johnson LC, Tepas DJ, Colquhoun WP, Colligan MJ (eds) Biological rhythms and shift work. Spectrum, New York, pp 271–282Google Scholar
  39. 39.
    Prokop P, Prokop L (1955) Ermüdung und Einschlafen am Steuer. Verkehrsmed 1: 19–30Google Scholar
  40. 40.
    Kogi K, Ohta T (1975) Incidence of near accidental drowsing in locomotive driving during a period of rotation. J Hum Ergol 4 (65–76)Google Scholar
  41. 41.
    Åkerstedt T, Torsvall L, Fröberg JE (1983) A questionnaire study of sleep/wake disturbances and irregular work hours. Sleep Res 12: 358Google Scholar
  42. 42.
    Tune GS (1969) Sleep and wakefulness in a group of shift workers. Brit J Industr Med 26: 54–58PubMedGoogle Scholar
  43. 43.
    Bjerner B, Holm Å, Swensson Å (1948) Natt och skiftarbete (Night and shift work). Statens Offentliga Utredningar, StockholmGoogle Scholar
  44. 44.
    Åkerstedt T, Gillberg M (1981) Sleep disturbances and shift work. In: Reinberg A Vieux N, Andlauer P (eds) Night and shift work: biological and social aspects. Pergamon Press, Oxford, pp 127–138Google Scholar
  45. 45.
    Coleman RM, Dement WC (1986) Failing asleep at work: a problem for continuous operations. Sleep Res 15: 265Google Scholar
  46. 46.
    Kogi K (1981) Comparison of resting conditions between various shift rotation systems for industrial workers. In: Reinberg N, Vieux N, Andlauer P (eds) Night and shift work - biological and social aspects. Pergamon Press, Oxford, pp 155–160Google Scholar
  47. 47.
    Åkerstedt T (1977) Inversion of the sleep wakefulness patern: effects on circadian variations in psychophysiological activation. Ergonomics 20: 459–474PubMedCrossRefGoogle Scholar
  48. 48.
    Åkerstedt T, Fröberg JE, Levi L, Torsvall L, Zamore K (1977) Shift work and well-being. Laboratory of Clinical Stress Research, Karolinska Institute, StockholmGoogle Scholar
  49. 49.
    Fröberg JE, Karlsson CG, Levi L (1972) Shift work. A study of catecholamine excretion, self-ratings and attitudes. Studia Laboris Salutis 11: 10–20Google Scholar
  50. 50.
    Dahlgren K (1981) Long-term adjustment of circadian rhythms to a rotating shiftwork schedule. Scand J Work Environ Health 7: 141–151PubMedCrossRefGoogle Scholar
  51. 51.
    Chaumont AJ, Laporte A, Nicolai A, Reinberg A (1979) Adjustment of shift workers to a weekly rotation. Chronobiologia 6 (suppl): 27–36Google Scholar
  52. 52.
    Folkard S, Monk TH, Lobban MC (1978) Short and long term adjustment of circadian rhythms in “permanent” night nurses. Ergonomics 21: 785–799PubMedCrossRefGoogle Scholar
  53. 53.
    Patkai P, Åkerstedt T, Pettersson K (1977) Field studies of shift work. I. Temporal patterns in psychophysical activation in permanent night workers. Ergonomics 20: 611–619PubMedCrossRefGoogle Scholar
  54. 54.
    Wilkinson R, Allison S, Feeney M, Kaminska Z (1989) Alertness of night nurses: two shift systems compared. Ergonomics 32: 281–292PubMedCrossRefGoogle Scholar
  55. 55.
    Torsvall L, Åkerstedt T (1987) Sleepiness on the job: continuously measured EEG changes in train drivers. Electroencephalogr Clin Neurophysiol 66: 502–511PubMedCrossRefGoogle Scholar
  56. 56.
    Daniel RS (1967) Alpha and theta EEG in vigilance. Percept Mot Skills 25: 697–703PubMedCrossRefGoogle Scholar
  57. 57.
    O’Hanlon JF, Beatty J (1977) Concurrence of electroencephalographic and performance changes during a simulated radar watch and some implications for the arousal theory of vigilance. In: Mackie RR (ed) Vigilance. Plenum Press, New York, pp 189–202Google Scholar
  58. 58.
    Torsvall L, Åkerstedt T, (1988) Extreme sleepiness: quantification of EOG and spectral EEG parameters. Int J Neurosci 38: 435–411PubMedCrossRefGoogle Scholar
  59. 59.
    Åkerstedt T, Torsvall L, Gillberg M (1985) Sleepiness in laboratory and field experiments. In: Koella WP, Riither E, Schulz H (eds) Sleep 1984. Gustav Fischer Verlag, Stuttgart, pp 88–89Google Scholar
  60. 60.
    Caille EJ, Bassano JL (1977) Validation of a behavior analysis methodology: variation of vigilance in night driving as a function of the rate of carboxyhemoglobin. In: Mackie RR (ed) Vigilance. Plenum Press, New York, pp 59–72Google Scholar
  61. 61.
    Fruhstorfer H, Lnaganke P, Meinzer K, Peter JH, Pfaff U (1977) Neurophysiological vigilance indicators and operational analysis of a train vigilance monitoring device: a laboratory and field study. In: Mackie RR (ed) Vigilance. Plenum Press, New York, pp 147–162Google Scholar
  62. 62.
    Haslam DR (1982) Sleep loss, recovery sleep, and military performance. Ergonomics 25: 163–178PubMedCrossRefGoogle Scholar
  63. 63.
    Lille F, Chéliout F (1982) Variations in diurnal and nocturnal waking state in air traffic controllers. Eur J Appl Physiol 49: 319–328CrossRefGoogle Scholar
  64. 64.
    Folkard S, Condon R, Herbert M (1984) Night shift paralysis. Experientia 40: 510–512PubMedCrossRefGoogle Scholar
  65. 65.
    Bjerner B, Holm Å, Swensson A (1955) Diurnal variation of mental performance. A study of three-shift workers. Br J Ind Med 12: 103–110PubMedGoogle Scholar
  66. 66.
    Brown RC (1949) The day and night performance of teleprinter switchboard operators. Occup Psychol 23: 121–126Google Scholar
  67. 67.
    Hildebrandt G, Rohmert W, Rutenfranz J (1974) 12 and 24 hour rhythms in error frequency of locomotive drivers and the influence of tiredness. Int J Chronobiol 2: 175–180PubMedGoogle Scholar
  68. 68.
    Tepas DI, Walsh JK, Moss PD, Armstrong D (1981) Polysomnographic correlates of shift worker performance in the laboratory. In: Reinberg A, Vieux N, Andlauer P (eds) Night and shift work: biological and social aspects. Pergamon Press, Oxford, pp 179–186Google Scholar
  69. 69.
    Tilley AJ, Wilkinson RT, Warren PSG, Watson WB, Drud M (1982) The sleep and performance of shift workers. Hum Factors 24: 624–641Google Scholar
  70. 70.
    Klein DE, Brüner H, Holtman H (1970) Circadian rhythm of pilot’s efficiency, and effects of multiple time zone travel. Aerosp Med 41 (125–132)PubMedGoogle Scholar
  71. 71.
    Monk TH, Folkard S (1985) Shiftwork and performance. In: Monk TH, Folkard S (eds) Hours of work. John Wiley, Chichester, pp 239–252Google Scholar
  72. 72.
    Lauber JK, Kayten PJ (1988) Sleepiness, circadian dysrhythmia, and fatigue in transportation system accidents. Sleep 11: 503–512PubMedGoogle Scholar
  73. 73.
    Harris W (1977) Fatigue, circadian rhythm and truck accidents. In: Mackie RR (ed) Vigilance. Plenum Press, New York, pp 133–147Google Scholar
  74. 74.
    Hamelin P (1987) Lorry driver’s time habits in work and their involvement in traffic accidents. Ergonomics 30: 1323–1333PubMedCrossRefGoogle Scholar
  75. 75.
    Ribak J, Ashkenzai IE, Klepfish A etal. (1983) Diurnal rhythmicity and airforce flight accidents due to pilot error. Aviat Space Environ Med 54: 1096–1099PubMedGoogle Scholar
  76. 76.
    Price W, Holley DC (1981) The last minutes of flight 2860: an analysis of crew shift work scheduling. In: Reinberg A, Vieux H, Andlauer P (eds) Night and shift work: biological and social aspects. Pergamon Press, Oxford, pp 287–298Google Scholar
  77. 77.
    Wegmann JM, Hasenclever S, Michel C, Trumbach S (1985) Models to predict operational loads to flight schedules. Aviat Space Environ Med 56: 27–32PubMedGoogle Scholar
  78. 78.
    Mitler MM, Czeisler CA, Dement WC, Dinges DF, Graeber RC (1988) Catastrophes, sleep and public policy. Concensus Report. Sleep 11: 100–109Google Scholar
  79. 79.
    Zulley J (1979) Der Einfluss von Zeitgebern auf den Schlaf des Menschen. Fischer, Frankfurt am MainGoogle Scholar
  80. 80.
    Czeisler CA, Weitzman ED, Moore-Ede MC, Zimmerman JC, Knauer RS (1980) Human sleep: its duration and organization depend on its circadian phase. Science 210: 1264–1267PubMedCrossRefGoogle Scholar
  81. 81.
    Knauth P, Rutenfranz J, Schulz H, et al. (1980) Experimental shift work studies of permanent night, and rapidly rotating, shift systems. II. Behaviour of various characteristics of sleep. Int Arch Occup Environ Health 46: 111–125PubMedCrossRefGoogle Scholar
  82. 82.
    Fröberg J, Karlsson CG, Levi L, Lidberg L (1975) Circadian variations of catecholamine excretion, shooting range performance and self-ratings of fatigue during sleep deprivation. Biol Psychol 2: 175–188PubMedCrossRefGoogle Scholar
  83. 83.
    Fröberg JE, Karlsson CG, Levi L, Lidberg L (1975) Psychological circadian rhythms during a 72-hour vigil. Försvarsmedicin II: 192–201Google Scholar
  84. 84.
    Åkerstedt T, Gillberg M, Wetterberg L (1982) The circadian covariation of fatigue and urinary melatonin. Biol Psychiat 17: 547–554PubMedGoogle Scholar
  85. 85.
    Carskadon MA, Dement WC (1981) Cumulative effects of sleep restriction on daytime sleepiness. Psychophysiology 18: 107–113PubMedCrossRefGoogle Scholar
  86. 86.
    Åkerstedt T, Gillberg M (1982) Experimentally displaced sleep: effects on sleepiness. Electroencephalogr Clin Neurophysiol 54: 220–226PubMedCrossRefGoogle Scholar
  87. 87.
    Åkerstedt T, Kecklund G, Hjerpe L (1989) Permanent night work as a complement to twoshift work: its effects on health and reasons for its attraction. Paper presented at the 9th Congress of Shift Work, Verona, ItalyGoogle Scholar
  88. 88.
    Koller M (1983) Health risk related to shift work. Int Arch Occup Environ Health 53: 59–75PubMedCrossRefGoogle Scholar
  89. 89.
    Wynn RF, Ryan GM, Cullen IH (1986) Adjustment to shiftwork and its prediction. In: Haider M, Koller M, Cervinka R (eds) Night and shiftwork: longterm effects an their prevention. Peter Lang, Frankfurt am Main, pp 101–108Google Scholar
  90. 90.
    Dumont M, Montpaisir J, Infant-Rivard C (1987) Past experience of nightwork and present quality of life. Sleep Res 16: 40Google Scholar
  91. 91.
    Guilleminault C, Czeisler S, Coleman R, Miles L (1982) Circadian rhythm disturbances and sleep disorders in shift works. In: Buser PA, Cobb WA, Okuma T (eds) Kyoto Symposia (EEG Supl no. 36 ). Elsevier, Amsterdam, pp 709–714Google Scholar
  92. 92.
    Home JA, Östberg O (1976) A self-assessment questionnaire to determine morningness-eveningness in human circadian rhythms. Int J Chronobiol 4: 77–110Google Scholar
  93. 93.
    Torsvall L, Åkerstedt T (1980) A diurnal type scale. Scan J Work Environ Health 6: 283–290CrossRefGoogle Scholar
  94. 94.
    Åkerstedt T, Torsvall L (1981) Shift work. Shift-dependent well-being and individual differences. Ergonomics 24: 265–273PubMedCrossRefGoogle Scholar
  95. 95.
    Costa G, Lievore F, Casaletti G, Gaffuri E, Folkard S (1989) Circadian characteristics influencing interindividual differences in tolerance and adjustment to shift work. Ergonomics 32: 373–385PubMedCrossRefGoogle Scholar
  96. 96.
    Folkard S, Monk TH, Lobban MC (1979) Towards a predictive test of adjustment to shift work. Ergonomics 22: 79–91PubMedCrossRefGoogle Scholar
  97. 97.
    Minors DS, Waterhouse JM (1985) Circadian rhythms in deep body temperature, urinary excretion and alertness in nurses on night work. Ergonomics 28: 1523–1530PubMedCrossRefGoogle Scholar
  98. 98.
    Härmä MI, Ilmarinen J, Knauth P, Rutenfranz J, Hanninen O (1986) The effect of physical fitness intervention on adaptation to shiftwork. In: Haider M, Roller M, Cervinka R (eds) Night and shift work: longterm effects and their prevention. Peter Lang, Frankfurt am Main, pp 221–228Google Scholar
  99. 99.
    Lavie P (1981) Sleep habits and sleep disturbances in industry workers in Israel: main findings and some characteristics of workers complaining of excessive daytime sleepiness. Sleep 4: 147–158PubMedGoogle Scholar
  100. 100.
    Åkerstedt T (1985) Adjustment of physiological circadian rhythms and the sleep-wake cycle to shift work. In: Monk TH, Folkard S (eds) Hours of work. John Wiley, Chichester, pp 185–198Google Scholar
  101. 101.
    Walker J (1985) Social problems of shift work. In: Monk TH, Folkard S (eds) Hours of work. John Wiley, Chichester, pp 211–225Google Scholar
  102. 102.
    Rutenfranz J, Haider M, Kolier M (1985) Occupational health measures for night workers and shift workers. In: Monk TH, Folkard S (eds) Hours of work. John Wiley, Chichester, pp 199–210Google Scholar
  103. 103.
    Åkerstedt T, Gillberg M, Torsvall L (1989) Napping and shift work. In: Dinges DF, Broughton R (eds) Sleep and alertness: the nature of napping. Raven Press, New York, pp 205–220Google Scholar
  104. 104.
    Lisper HO, Laurell H, von Loon J (1986) Relation between time of falling asleep behind the wheel on a closed track and changes in subsidiary reaction time during prolonged driving on a motorway. Ergonomics 29: 445–453PubMedCrossRefGoogle Scholar
  105. 105.
    Ehret CF (1981) New approaches to chronohygiene for the shift worker in the nuclear power industry. In: Reinberg A, Vieux N, Andlauer P (eds) Night and shift work: biological and social aspects. Pergamon Press, Oxford, pp 263–327Google Scholar
  106. 106.
    Walsh JK, Muehlbach MJ, Schweitzer PK (1984) Acute administration of triazolam for the daytime sleep of rotating shift workers. Sleep 7: 223–229PubMedGoogle Scholar
  107. 107.
    Walsh JK, Sugerman JL, Schweitzer PK, Duntley S (1986) The use of triazolam in simulated shift work. II. Sleep tendency and performance. Sleep Res 15: 46Google Scholar
  108. 108.
    Bonnet MH, Mitler M, Gillin JC, Mendelson W (1986) Triazolam, sleep satiation, and nocturnal work shift sleepiness and performance. Sleep Res 15: 28Google Scholar
  109. 109.
    Eastman CI (1987) Improving adaptation to non-24-hr sleep schedules with bright light. Sleep Res 16: 14Google Scholar

Copyright information

© Springer-Verlag Berlin Heidelberg 1991

Authors and Affiliations

  • T. Åkerstedt
    • 1
  1. 1.IPM and Department of Stress ResearchKarolinska InstituteStockholmSweden

Personalised recommendations