Oh sleep! It is a gentle thing,
Beloved from pole to pole!
To Mary Queen the praise be given!
She sent the gentle sleep from Heaven,
That slid into my soul.
(Coleridge. The Rime of the Ancient Mariner)

In Idle Theory, all living creatures are regarded as alternating between busy and idle states. And usually, although not always, the idle state entails expending less energy.

During sleep between episodes of work, creatures slow down or switch off some activities, and consequently expend less energy. And this reduction of energy expenditure increases their idleness, because it reduces the amount of time they need to work to acquire energy. And in the case of hibernating animals, it seems that very substantial reductions in energy expenditure are achieved, allowing hibernating animals to 'sleep' for months. An ability to sleep increases idleness, and the likelihood of survival.

And in the wider natural world, it seems that many animals alternate fairly rapidly between sleeping and waking. And some cats seem able to sleep for 23 hours a day.

When asleep, humans are almost entirely inactive, and their metabolic rate falls to something like two thirds of daytime resting metabolic rate (one result of which is that they generally need thicker insulating covers or duvets on their beds at night than the clothing they wear during the day).

And thus sleep would seem to be an idle time activity. In fact, it is often held up as the epitome of idleness.

But, at least in the case of humans, sleep does not appear to be entirely an optional state of extreme relaxation. Sleep does not seem to be something that humans can take or leave as they choose. Humans need to sleep: if they have not slept for days, they become disoriented and unable to carry out even simple tasks. Even the inmates of Auschwitz were permitted to sleep at night. It seems that it is not simply that sleep is an optional way of passing idle time, but rather that it is an activity that is somehow necessary for the maintenance of human life.

The human need for sleep seems to vary with age. Young children may sleep for 12 hours a night. Adults usually need 7 or 8 hours. But the elderly quite often need only 4 or 5 hours.

And if sleep is a necessary activity, then it might be argued that sleep is something that people 'busy' themselves at performing. During sleep, time appears to pass rapidly, which is the same experience as being busy at work.¹

And this creates something of a problem for Idle Theory, in that if sleep is an 8 hour idle time activity, then human idleness can never fall below 33% idle, because there one third of the day will always be devoted to idly sleeping, whatever the circumstances. And if sleep is a busy time activity, then human idleness can never rise above 66% idle, because people will always be busy sleeping for one third of the day, however idle they are the rest of the time.

Indeed, sleep would seem to introduce a third state into Idle Theory - a state of "necessary idleness" or "busy idleness" or "idle busyness" - to set beside the busy and idle states. So that instead of a duality of alternating busyness and idleness, there is instead perhaps a trinity of busyness, idleness, and sleep.