The body has a daily internal cycle called the circadian rhythm. This cycle, characterised by daily variations in biological markers such as cortisol, glucose, body temperature, insulin, and melatonin, creates a temporal predisposition for resting and waking.
The advantage of having such an internal cycle is that it allows the body to “orchestrate physiological changes that lead, rather than lag, the daily change in environment” [4]. This means your body can start preparing you to wake up several hours before the sun begins to rise. The adaptive utility of having a circadian cycle has been demonstrated even in bacteria. Strains that have had their circadian cycle genetically removed have been shown to be outcompeted by those with a functioning rhythm.