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While I was at lecture last Thursday, learning about corollary discharges and the discomfort that occurs when signals from sensory and inter neurons disagree, my friend was fast asleep in my room. He had not intended to spend a night in Bryn Mawr, but his sleep cycle prevented him from even being alert enough to navigate public transit home. He has a combination of two different conditions that affect the body's internal clock, or circadian rhythm: Delayed Sleep Phase Syndrome (DSPS) and Non 24-hour Sleep-Wake Syndrome.

Delayed sleep phase syndrome means being physically unable to fall asleep until very late hours at night, and unable to wake up until late morning or afternoon. This would be fine if all jobs and classes were flexible enough to accomodate all sleep schedules, but the U.S. generally runs on a 9-5 schedule, which may be impossible to keep for a person with DSPS. Also, people with "normal" circadian rhythm patterns can compensate for a lack of sleep the night before by going to bed earlier the next night, thus "resetting" the internal clock. A person with DSPS may not be able to sleep until whatever their "normal" bedtime is, even if they are sleep deprived. (See wikipedia's entry on dsps for more info).

Non-24 hour sleep-wake syndrome is a more serious condition, in which the person's brain and body insist that the day is longer than 24 hours. In practice, this amounts to gradual delays in sleeping and waking time over the course of days, until the person's "clock" has finally run its cycle. (This cycle, when left unchecked, is known as free running). Again, this cycle doesn't work particularly well in a world that demands 9-5 work or class in most cases.(See wikipedia's article on non 24-hour sleep-wake syndrome for more info).

So where do central pattern generators and corollary discharges fit into this picture? Given the explanation of motion sickness last Thursday, it seems to me that you could probably apply a similar model to circadian rhythm disorders. In DSPS and Non 24-Hour Sleep-Wake syndrome, the central pattern generators in the body, probably in combination with hormones from the endocrine system (like melatonin), register the feeling of tiredness during daylight hours. But their sensory neurons register sunlight, light sky, people moving-all obvious signs of day, a time when one shouldn't be asleep. Maybe the disconnect between these inputs is one factor that makes circadian rhythm disorders so difficult to manage.

Also: Not ever country runs on a 9-5 clock like the US does. In a country where keeping later hours is the norm, would a person be diagnosed with DSPS if, say, they went to bed at 11 and woke up at 7 or 8, a time frame that would be considered typical or desirable in the US? The disconnects between inputs would be fundamentally the same, but the desirable time frame varies so much. Does DSPS even exist as a diagnosis in other parts of the world?