The afternoon pressure for sleep

Halfway through the waking period, around early afternoon, there is a strong propensity for sleep to occur again. This second sleep tendency is part of a 12-hour rhythm of sleepiness, that I have termed "The Afternoon Pressure for Sleep" (APS). Evidence for this bicircadian sleepiness, include siesta cultures, short sleep latencies in the afternoon, a midday peak in pathological sleep attacks, and the concentration of napping to this afternoon window. The flexible nature of the human sleep/wake cycle allows for napping to be easily suppressed, as necessitated by social and occupational demands, and so the APS is readily masked. [Continues.]