Working during the night leads to misalignment between the endogenous circadian timing system and timing of sleep and wake, disturbing multiple physiological processes in the human body [1]. This misalignment typically results in a shorter, less consolidated, and highly disrupted sleep during the daytime, and increased sleepiness and performance decrements during night shifts [2, 3]. Indeed, night workers typically report shorter sleep durations than their day worker counterparts, resulting in greater levels of sleepiness on the following night shift, with accumulating sleep loss across multiple night shifts [4]. Sleep insufficiency is the most widespread problem affecting shift workers [5, 6]. When sleep insufficiency is severe and associated with excessive sleepiness during wake, it can even lead to Shift Work Disorder [7]. Shift workers report various sleep patterns, showing stable inter-individual differences [8], and are related to time of day and number of night shifts [9, 10]. Observational research on sleep strategies in shift workers’ natural setting are still limited [10], with most prior reports being either laboratory-based [11–13] or retrospective [14]. The aim of this study was to observe different sleep strategies used by actual night shift workers between consecutive night shifts to gain a deeper understanding of the strategies in terms of duration, quality, timing, and regularity of their sleep.

Forty-three shift workers completed the two-week study; of these, four participants were excluded due to having not worked any night shifts during the two weeks, leaving N=39. Of these, four participants showed a major discrepancy in the number of consecutive night shifts they reported at screening versus what they did during their study (>5 night deviance) and therefore were excluded from further analysis (deemed unreliable). Finally, two participants reported only two consecutive night shifts (with one intervening sleep episode) during the 14 days and were therefore excluded.

The final participants included 23 women and 10 men, aged 40 ± 15 years (mean ± SD). On average, the participants worked five night shifts per week and had 11 years of shift work experience. Eighteen participants worked in healthcare, six in hospitality, three in public safety, two in office work, one in retail, and three in other sectors. Night shifts were identified when work started between 18:00–24:00 and lasted at least eight hours. Overall, a total of 206 sleep episodes recorded in the daily surveys were identified as occurring between a series of at least two consecutive night shifts. Six sleep episodes between night shifts were analysed on average, with a range of 2–11 per participant.

The 33 shift workers were categorized into four groups based on their sleep strategy using their day-to day sleep starting times and number of sleep bouts between consecutive night shifts. We classified seven as morning sleepers, eight as delayed sleepers, eight as split sleepers, and ten as mixed sleepers. See Table 1 for a detailed description of participant demographic characteristics.

The timing and duration of sleep episodes between consecutive night shifts in the four sleep strategy groups are presented in Figure 1. As expected, the timing of the first sleep bout (i.e. start time) differed significantly between groups (H=18.70, p<0.001). Pairwise comparisons showed that the sleep start time of the morning group was significantly earlier than the delayed group (p<0.001), and earlier than the mixed sleepers (p=0.043). The start time of the delayed group was later than the split sleepers (p<0.002) and the mixed sleepers (p=0.015). No differences were found in the regularity of start time of the first sleep episode between groups (H)=6.24, p=0.100). TST between consecutive night shifts showed no significant differences between groups (Welch_(3,16)=2.18 p=0.131), although regularity of TST did differ significantly between the groups (H=12.38, p=0.006). Mixed sleepers showed more irregularity compared to delayed (p=0.008) or split sleepers (p=0.001). Subjective soundness of sleep was not different between groups (H=0.54, p=0.910), but feeling of being refreshed from sleep did differ between the groups (F_(3,29)=3.70, p=0.023). Delayed and mixed sleepers reported feeling more refreshed than split sleepers (delayed: p=0.004; mixed: p=0.042), and delayed sleepers felt more refreshed than morning sleepers (p=0.046).

We studied a small sample of night shift workers in their natural setting, revealing a variety of sleep patterns between consecutive night shifts. We identified morning, delayed, and split-sleep strategies also reported in previous studies [5, 10, 11, 15]. In addition, we identified a subgroup who showed a mix of sleep strategies (or lack of a consistent strategy) between consecutive night shifts. We found that overall sleep duration was shorter than the recommended 7–9 hours daily, regardless of which strategy used between night shifts, congruent with previous studies [5, 9, 10].

We observed similar sleep durations between the various sleep strategies used in these shift workers, consistent with two laboratory-based studies performed on healthy men [11, 12]. Other simulated shift work studies reported that scheduling delayed sleep between night shifts could lengthen sleep duration and improve performance and adaptation on night shifts in both younger [16, 17] and older adults [18, 19]. Here, delayed sleepers reported feeling more refreshed than split sleepers. We found no differences in individual variability of sleep start times between groups, but individual variability in TST was greater for mixed-sleep shift workers than delayed- or split-sleep shift workers. This result raises the question of whether mixed sleepers have any strategy at all.

A strength of this study is that experienced shift workers were studied when following their usual sleep patterns, which enhances external validity of the results. However, recruitment took place during the COVID-19 pandemic, which could have both limited enrollment and created a selection-bias among those who did participate. Other limitations are the small sample size and imbalance in sex distribution, as well as lack of detail about second jobs or why each individual chose their particular sleep strategy. Also, we did not screen for use of medication, sleep disorders or medical conditions, which could have influenced our results. Even though actigraphy would be a better approach for real-time monitoring of sleep patterns, daily electronic surveys were used to limit wrong data entries by hand and were easy to use.

Future studies where workers are followed longer and queried at regular intervals, together with actigraphy, should be carried out to better understand factors contributing to individual sleep strategies. Although we did not explore sleep behavior during night shifts (such as napping during breaks) or while transitioning onto or off of a series of night shifts, limiting our investigation to sleep between consecutive night shifts gave us a way to compare sleep between shift workers, especially knowing that their schedules differed in terms of shift type, duration, time of day, rotation, occupation and workload on any given shift. Further studies of strategies used by different shift workers when rotating onto and off of night shifts would be useful to understand the prevalence of different strategies and whether those result in differences in sleep duration and quality during such transitions. Lastly, given the findings from laboratory studies [18, 19], it would be interesting to investigate how different self-selected sleep strategies relate to sleepiness during the night shifts [11, 14].

To conclude, our results provide insight into the various sleep patterns adopted by shift workers when working consecutive night shifts, and these findings may help design new sleep interventions that can be individualized to improve shift workers’ sleep, safety, and health.