Objectives Sleepiness is a specific aspect of fatigue and considered a key factor linking working time to health and safety outcomes, such as occupational injuries. Therefore, this meta-analysis synthesizes evidence on how specific work schedule characteristics relate to sleepiness. Specifically, we examine how sleepiness varies with: (a) shift type (eg, day versus night), (b) shift length, and (c) the number of consecutive shifts.

Methods We conducted a systematic literature search across multiple platforms and databases. Only studies that measured sleepiness using the Karolinska Sleepiness Scale were included. Mean effect sizes (Hedges` g) were calculated using random-effects models.

Results The analysis included 48 studies (28 on shift type, 30 on shift length, and 13 on shift number). Compared to day shifts, sleepiness was significantly higher during night shifts and lower during afternoon shifts. Sleepiness increased with shift length during night shifts but showed no consistent pattern during day shifts. Furthermore, sleepiness was highest on the first night shift and decreased over subsequent nights, whereas it remained relatively stable across consecutive day shifts. Due to the low number of studies, results on shift length and shift number were limited or unavailable for afternoon shifts.

Conclusions Overall, this meta-analysis shows that sleepiness is most pronounced during night shifts, particularly the first in a block. These findings emphasize the importance of circadian and homeostatic factors in shift work. Therefore, shift schedules should be designed to mitigate these heightened risks, for example by allowing sufficient recovery time before and during the first night shift.

Key terms fatigue; meta-analysis; night work; occupational health; shift work; sleepiness; work recovery; work schedule; working time arrangement