Summary In most situations, complete circadian adjustment is not recommended for night workers. With complete adjustment, workers experience circadian misalignment when returning on a day‐active schedule, causing repeated circadian phase
shifts and internal desynchrony. For this reason, partial circadian realignment was proposed as a good compromise to stabilize internal circadian rhythms in night shift workers. However, the extent of partial circadian adjustment necessary to improve sleep and vigilance quality is still a
matter of debate. In this study, the effects of small but statistically significant partial circadian adjustments on sleep and vigilance quality were assessed in a laboratory simulation of night work to determine whether they were also of clinical significance. Partial adjustments obtained
by phase delay or by phase advance were quantified not only by the phase shift of dim light salivary melatonin onset, but also by the overlap of the episode of melatonin production with the sleep–wake cycle adopted during simulated night work. The effects on daytime sleep and night‐time
vigilance quality were modest. However, they suggest that even small adjustments by phase delay may decrease the accumulation of sleep debt, whereas the advance strategy improves subjective alertness and mood during night work. Furthermore, absolute phase shifts, by advance or by delay, were
associated with improved subjective alertness and mood during the night shift. These strategies need to be tested in the field, to determine whether they can be adapted to real‐life situations and provide effective support to night workers.