Narcoleptics have messed up sleep-wake cycles. According to
this page from the NIH,
Quote:
Narcolepsy is a chronic neurological disorder caused by the brain's inability to regulate sleep-wake cycles normally. At various times throughout the day, people with narcolepsy experience fleeting urges to sleep. If the urge becomes overwhelming, individuals will fall asleep for periods lasting from a few seconds to several minutes.
...
The cause of narcolepsy remains unknown. It is likely that narcolepsy involves multiple factors interacting to cause neurological dysfunction and sleep disturbances.
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I think this makes Sharpe a napper. One article I read described it as the feeling you get when you have missed an entire night of sleep, and you start spontaneously drifting off even as you struggle to stay alert.