If I were to design a laboratory experiment that reliably disturbed sleep and increased fatigue for eight to ten days, it would look a lot like the festive period. To understand why, you need to consider how normal sleep works.
Sleep is controlled by three things: the supply-and-demand relationship between sleep and wakefulness or ‘homeostasis’ – in a nutshell, the longer we stay awake, the greater the pressure to sleep; the body clock (or circadian rhythm) which syncs our sleep-wake cycles with the 24-hour day so that we feel sleepier during darkness than during light; and psychological factors that calm us down in readiness for sleep.
Normally, these processes work in harmony. So we are most likely to fall asleep when sleep pressure is high, our circadian rhythm tells us it’s bedtime and our minds are calm.
Among otherwise good sleepers, events that disrupt sleep tend to affect just one of these factors at a time, and then only briefly, allowing us to take the effects in our stride.