ABSTRACTDaytime workers tend to have shorter sleep duration and earlier sleep onset during work days than on days off. Large individual differences in sleep onset and sleep duration may be observed on work days, but work usually synchronizes sleep offset to a similar time. The present study describes individual differences in sleep behaviour of 48 daytime workers (25 men, aged 20â58 years) from an iron ore mine in Northern Sweden. The aim of the study was to determine whether differences in sleep patterns during work days were associated with the outcomes of sleepiness and sleep complaints. Cluster analysis was used to group workers into two categories of sleep onset and sleep duration. The âLate Sleep Onsetâ cluster comprised workers who slept 1.30 h later than the âEarly Sleep Onsetâ cluster (p < 0.0001 for all weekdays). The âLong Sleep Durationâ cluster slept 1.10 h longer than the âShort Sleep Durationâ cluster (p < 0.0002 for work nights). The âLate Sleep Onsetâ cluster reported less refreshing sleep (p < 0.01) and had lower sufficient sleep scores (p < 0.01) than the âEarly Sleep Onsetâ cluster. The âShort Sleep Durationâ cluster also reported lower scores for sufficient sleep (p < 0.04) than the âLong Sleep Durationâ cluster. For combined characteristics (phase and duration), workers with a late phase and short sleep duration reported greater sleep debt and sleepiness than workers with an early phase and short sleep duration (p < 0.02). Work schedule and commuting time modulate both sleep phase and sleep duration independently. Workers, classified as having an intermediate sleep phase preference, can organize their sleep time in order to minimize sleep debt and sleepiness symptoms. Individual differences in sleep phase and duration should be considered when promoting well-being at work even among groups with similar sleep needs. In order to minimize sleep debt and sleepiness symptoms, successful sleep behaviour could be promoted involving extend use of flexitime arrangement (i.e. later starting times) and reduce use of alarm clocks.