Sleep health is a multidimensional construct that includes parameters beyond sleep duration, such as timing and regularity. The stability of the sleep-wake schedule over time is a particularly important factor contributing to health. Although circadian rhythm disruption is associated with poor mental health, the role of sleep variability remains unclear.
The first year of medical training (residency) is a rare circumstance marked by a sharp increase in workload and changing schedules that span 24 hours a day.
Furthermore, the prevalence of depression increases dramatically after its onset. Therefore, it may act as a prospective model to better understand the relationship between sleep variability and mood in a broader population.
Technological advances allow sleep to be objectively measured through passive recording, in real time, with minimal user expense or burden. Multisensory and wrist-based sleep tracking devices provide estimates of sleep patterns over extended periods of time in individuals under demanding circumstances such as medical training.
Additionally, mobile platforms allow real-time entry of self-reported symptoms. Therefore, the use of technology allows sleep to be more completely characterized, while mood is assessed, to identify specific sleep disturbances that contribute to depression.
Using a sample of more than 2000 subjects and a multisensory sleep tracking device, we sought:
1) Characterize changes in objective sleep, monitored during the transition through the internship.
2) Identify specific objective sleep characteristics associated with depression over the course of the internship year.
3) Assess the impact of daily changes in objective sleep duration and sleep-wake time on next-day mood.
It was hypothesized that decreased sleep duration and greater variability in sleep-wake time during this transition would be associated with lower mood and more depressive symptoms.
| Results |
The study cohort consisted of 2115 trainees (56% female; age 27.5 ± 2.4 years). The Patient Health Questionnaire (CSP-9) was applied at the beginning and during the internship, with a score ≥ 10 defining symptoms of depression.
With the onset of internship stress, physicians experienced a significant reduction in total sleep time (TTS) of 24 h (17 min) and an advance in sleep rhythm, going to bed about half an hour later. Furthermore, there was a significant increase in the standard deviation (SD) of sleep duration (16 min) and time (bedtime, 1 h 53 m; wake time, 1 h 30 min) with the transition to the year of practices.
Mean CSP-9 scores during the internship year ranged from 0 to 25.5. On average, for every hour of decrease in 24-h TTS, the CSP-9 score worsened by 0.11 points. An even larger effect was observed for sleep duration variability; For every hour increase in the SD of the 24-h TTS, the CSP-9 worsened by 0.4 points. Median bedtime was associated with depression; the later the onset of sleep, the more depressive symptoms.
Greater variability in wake time was also associated with higher depressive symptom scores. When all factors were taken together, lower mean 24-h TTS and bedtime variability and higher variability in 24-h TTS and wake time were associated with higher depressive symptom scores.
Overall, variability in sleep measures and mean levels of sleep measures had similar predictive value for depressive symptom scores.
Of 2115 subjects, 358 had average CSP-9 scores during internship above depression criteria (≥10). Compared with the remaining 1757 nondepressed subjects, they did not differ significantly in the mean or median of any sleep measure, but had significantly greater variability in them.
On the other hand, the increase in 24-h TTS on the previous day and subsequent wake time was associated with an improvement in mood the next day. In contrast, going to bed later was associated with worse mood the next day. Furthermore, variability in 24-h TTS and time awake was associated with decreased mood the following day. Variability in bedtime between nights did not show a significant impact on mood.
| Discussion |
This research revealed that in medical trainees, reduced total sleep hours and delayed bedtime, and even more prominently, greater variability in total sleep and wake time, were associated with increased depressive symptoms .
On a daily basis, reduced sleep duration, going to bed later, waking up earlier, and large changes in total sleep time and wake time were detrimental to next-day mood.
Intraindividual variability (VII) quantifies the daily variation in the mean of sleep parameters measured over multiple days, and higher VII can exert a negative impact on a variety of outcomes. The extreme work circumstances imposed on interns provide a model to comprehensively assess the impact of sleep variability on mood, which may be difficult to capture in the general population.
As stated, shorter sleep duration was associated with an increase in depression scores (CSP-9) during the internship year. This extends previous findings that demonstrated that short sleep duration is associated with depression in physicians in training. However, variability in sleep duration demonstrated an even stronger influence on the CSP-9 score, with a strong relationship between the SD of sleep duration and depression scores, despite adjustment for the TTS of 24 h.
Regarding sleep time, bedtime but not wakeup time was associated with depression. This may indicate that sleep onset insomnia or evening chronotype was associated with worse mood during internship, given the known association between delayed sleep-wake phase disorder and depression. However, after adjusting for sleep duration, this association was no longer significant and suggests that sleep loss is a potential factor underlying this finding.
Greater variability in wake time was associated with worse depression scores while, conversely, increased variability in bedtime improved scores. It should be considered that bedtime is more dependent on individual selection or biological propensity, while wake time is set by external and specific demands of this population, variable depending on the workload.
In the general population, this concept is highlighted by social jet lag, which describes the pattern of longer sleep duration on days off than on work or school days, being more pronounced in individuals with an evening circadian preference.
One hypothesis to explain the association of better depression scores with more variable sleep timing is that, in people who do not successfully modify their bedtime, greater variations in wake time result in more variable sleep durations ( and short), which is detrimental to the mood.
In contrast, people who successfully vary their bedtimes in response to changes in wake times maintain more stable and longer sleep durations, and therefore better mood.
The next day, mood worsened due to shorter sleep duration, waking up earlier, and going to bed later. Controlling for previous day’s sleep duration, sleep timing, and mood, changes in total sleep time and wake time were also associated with reduced mood the following day. Changes in bedtime did not impact mood the next day, suggesting that these changes are relevant to mood only in the context of their effect on sleep duration.
The findings support the conclusion that various measures of sleep may be more detrimental to mental health (and other conditions) than insufficient sleep alone, potentially through circadian disruption. Alertness and sleep are optimal in quality and duration when wakefulness is attempted during the time of high circadian alertness and sleep coincides with the period of pineal melatonin secretion and reduced core body temperature.
When external forces dictate behavioral rhythms out of alignment with the endogenous circadian rhythm, sleep and mood deteriorate. The detriment of circadian mood disturbances is evident in shift workers, who suffer the most profound and chronic manifestation of circadian misalignment.
It is important to consider as a limitation of this study that although the temporal relationship between sleep variability and depression may be valuable, potential unmeasured factors, such as physical activity and caffeine consumption, may prevent drawing conclusions about causality. Future randomized trials will definitively evaluate whether decreasing sleep variability reduces depression.
These findings provide a necessary foundation to inform institutional programming structures and guide self-management measures to improve sleep and circadian alignment within the confines of a demanding workload with the ultimate goal of optimizing mental health.
Our society today is connected on a global scale, offering work and social networking opportunities 24 hours a day, often at the expense of sufficient and consistent sleep.
Therefore, even in the context of small effect sizes, these findings have clinical value. By identifying variability in sleep duration and timing as a potential factor associated with mood, this modifiable behavior could be considered more broadly as part of a multifaceted approach to optimizing mental health in general adult populations.
