When Is Enough, Enough?

Abstract

Most biological needs can be taken in excess of actual requirement, but more for enjoyment, as in our eating without hunger but with a ‘hearty appetite’, and in drinking for pleasure, without thirst. Similarly, we can sleep in excess of its need, for enjoyment, reflecting an ‘appetite for sleep’ rather than a ‘sleep hunger’. That sleep quality is as important as its quantity is seen with the interactions between the circadian rhythm and sleep intensity, and in recovery sleep after total sleep loss, when less than half of the lost sleep needs to be reclaimed (mostly as deep, slow wave sleep -SWS). Sleeping once a day, as a single uninterrupted night-time block, is a more recent custom, when it was common to split this into ‘first’ and ‘second’ sleeps, when people would wake in the middle of the night, for various tasks then return to sleep. In striving for an unbroken night’s sleep, as is usual nowadays, this might even be somewhat ‘unnatural’. Then there is the short but useful daytime nap, or a more regular, longer siesta, that can be more efficient sleep-wise, in resulting in less total daily sleep need. All of which suggest that the recent concept of ‘social jetlag’, similar to sleep debt, may not be quite so problematic as is thought.