I read the article “Changes in sleep pattern and sleep quality during COVID-19 lockdown” by Gupta et al., with interest. The authors describe a significant worsening in the quality of sleep, delayed bedtime, delayed sleep onset, reduction in night-time sleep duration, and increased daytime napping in the study population. The authors have attributed some of these observations to disruption of the circadian system.
It might be fruitful to elaborate upon some of these observations on the basis of the principles of circadian regulation of sleep. There was noted a significant increase in the number of individuals going to bed after 11 PM during the lockdown – 610 (65.2%) as compared to before – 451 (48.4%). A corresponding reduction was seen in the number of individuals waking up before 6 AM from 246 (26.4%) before the lockdown to 183 (19.7%) after the lockdown. A total of 379 individuals reported going to bed later than before the lockdown and 406 waking up later than before the lockdown. These observations taken together indicate a significant shift to a later bedtime. What causes this shift? Two explanations can be proffered.
The two process model of sleep posits that the time of onset and maintenance of sleep are regulated by the interaction of homeostatic sleep pressure (Process S) and the circadian drive for wakefulness (Process C). The circadian system in turn is regulated – or entrained by zeitgebers (time-givers), most important of which is the environmental light–dark cycle. Light intensity of 5000–10,000 lux is thought to be required for entrainment of the circadian system. The intensity of sunlight on a bright day can reach up to 1,00,000 lux whereas indoor illumination rarely exceeds 500 Lux. Prolonged indoor confinement can significantly reduce sunlight exposure which can adversely impact the entrainment of the circadian system. In the absence of sufficient zeitgeber strength, individuals with a circadian period >24 h (who form the majority of human population) will gradually develop a phase delay of the circadian system which in turn can lead to a shift to later sleeping and waking times. In addition, insufficient sunlight exposure has been linked to poorer consolidation of night sleep and decreased quality of night sleep.
Social zeitgebers could also explain some of the study observations. Social zeitgebers include the schedule of daily work, school hours, commutes, mealtimes, etc. The present study observes a significant shift to later bedtimes and wake times in housewives and other nonmedical workers after the lockdown while such a prominent shift is not observed in the case of health-care workers – a majority of continued working outside home after lockdown [Table 4 – Gupta et al., 2020). There was also a significant reduction in number of people waking up with the help of alarm from 375 (40.2%) before the lockdown to 206 (22.2%) after the lockdown. The previously fixed working hours could have acted as the social zeitgeber for many individuals which had maintained earlier bedtimes and waking times – an effect lost during the lockdown.
It is, however, noteworthy that sleep is not a direct function of circadian rhythm and there are other factors also that are involved in the regulation of sleep. Furthermore, the degree to which home confinement can lead to reduction in sunlight exposure and whether the same would be significantly different from working in an indoor office environment remains a moot point.
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