The impact of returning to a daytime schedule on sleep, performance and mood after simulated fixed and rotating split shift schedules

Split shift schedules may be a suitable alternative to long shifts due to reduced accumulation of sleep pressure. However, returning to a daytime schedule (RTDS), performance and sleep deficits may occur as a result of changing the timing of sleep and wake periods. The current study investigated sleep, performance and mood during and following four 24h periods on fixed and rotating split shift schedules. Twenty four participants (10M, 21-36y) completed a 9-day laboratory study with two 10h baseline sleeps (22:00h-08:00h); one of three shift conditions: one of two 6h on / 6h off schedules, (Fixed A: 5h-time in bed (TIB) at 03:00h/15:00h, or Fixed B: 5h TIB at 09:00h/21:00h), or an 8h on / 8h off schedule (Rotating: 6h40 TIB); and RTDS with 10h TIB for 2-nights (22:00h-08:00h). The Fixed B condition had significantly less N3 sleep during RTDS compared to baseline (p<0.01). For all conditions, sleep onset latency, N2 onset latency and N3 onset latency were significantly longer during RTDS compared to baseline (p<0.05). Psychomotor vigilance was stable throughout the study. Subjective sleepiness (p<0.001) and positive affect (p<0.01) worsened during the shift schedule period but returned to baseline levels during RTDS. Negative affect increased during the shift schedule period and did not return to baseline levels upon RTDS (p<0.001). Although differences in sleep and negative affect were observed upon RTDS, the fixed and rotating shift schedules did not substantially worsen performance during the shift schedule or after RTDS in controlled laboratory conditions that are ideal for sleep.