Figure 2.

A catalog of aberrations concerning the timing of sleep. Red area indicates sleep time with the red circle showing the sleep start time for each condition. (a) Average sleep time for many people (Roenneberg et al., 2004). (b) A precisely reversed sleep-wake cycle relative to normal. This pattern has been reported rarely and often anecdotally. (c) With illnesses that are accompanied by fever, sleep duration often expands. Here, the timing of sleep symmetrically expands around the normal sleep time shown in (a). (d, e) Sleep timing can shift to an earlier or later phase. This would occur if the light-dark environment drastically and predictably were changed. (f) Sleep can become unconsolidated and fragmented (here, the indication of sleep onset is omitted due to multiple sleep episodes). Note that sleep timing is neither routinely documented in clinical practice nor in most surveys of sleep characteristics.