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Circadian rhythms are internally generated ~24hr cycles that drive daily oscillation in many aspects of physiology, from cardiac output to cognition. This system is entrained to the day-night cycle by light and is most sensitive to this synchronisation cue in the morning. Disruption to these circadian rhythms, through light exposure at the wrong time of day, such as late evening, or irregular sleep timing, are linked to various deleterious outcomes, including increased risk of neuropsychiatric disease.
Winter is something of a perfect storm for circadian disruption, combining shorter days (and therefore less outdoor light) with the changing of the clocks. Not only does the changing of the clocks in the UK from Standard Time (ST) to Daylight Saving Time (DST) represent acute circadian disruption, a bit like crossing a time zone. However, it also causes periods of chronic disruption where people experience sunlight later in the morning as sunrise happens later. As a result, the circadian clock is entrained to a later light stimulus, causing ‘phase delay’ i.e. individuals feeling the need to sleep later.
So what can be done? Mental health is potentially more vulnerable during the winter months, and this will vary from person to person due to genetic and environmental factors. Possible ways to mitigate this are:
1) engage in activities that support your mental health
2) reduce circadian disruption that may arise more during winter months due to aberrant light exposure
The latter can be achieved by having regular bed and wake up times, trying to see natural light in the morning and reduce light exposure before bed, using a SAD lamp if exposure to natural light is difficult, and reducing habits that might disrupt sleep (like caffeine late in the day).
The British Sleep Society recently published a statement about the issue of DST that discusses many aspects of the above in more detail.