No one will be surprised that our sleep-wake cycle is closely related to the circadian timing system. This first became obvious in time-isolation studies where subjects could go to sleep and rise as they wished, but without a clock on the wall to guide or remind them. Their sleep-wake cycle followed a free-running circadian pattern, not a 24-hour pattern. One person might go to sleep 30 minutes later each day, while another would vary the pattern by going to sleep even later each day, or sometimes earlier.
A structured 24-hour light dark cycle stabilizes this drift for most of us, yet circadian sleep disorders are common throughout the population. They can be complex to detect, since multiple factors influence when we go to bed, such as arbitrary work shifts, family responsibilities, irregular meal times, compromises with our bed partner’s wishes, getting too much light in the evening … and another simple biological factor, how long we’ve been awake.
Circadian rhythm sleep disorders come in a variety of patterns, falling asleep way too late, waking way too early, shifting sleep hours wildly from day to day, or sleeping later and later on successive nights (often with a sudden, unexpected switch earlier, which starts the cycle again). Different schedules of light therapy have been developed for the particulars of each disorder. In each case, the goal is to normalize sleep to the desired pattern, in which the patient can increase productivity and daytime energy, along with a lifting of depression in many cases.
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