Answer: The adaptive inactivity theory of sleep is an evolutionary-centric explanation of why organisms sleep.
Every living organism that we know of on Earth sleeps. Every creature has some behavioral pattern that repeats on a 24 hour cycle in synchrony with the rising and setting of the sun.
Interestingly, organisms have different sleep patterns depending on their ecological niche. Some animals, like humans, are most active in the day and least active at night. We have very refined visual systems that are effective in the light, which are great for stereoscopic ( three dimensional ) vision. However, we have poor night vision, making night activity especially dangerous. Creatures with this type of sleep-wake pattern are called diurnal animals.
The adaptive theory of sleep argues that each organism sleeps according to maximize their utility while they are awake, and minimize their energy expenditure when they are less capable of productivity. According to the adaptive theory of sleep, humans have evolved to sleep at night, during the time when we are minimally productive in order to conserve our energy.
Another aspect of the adaptive inactivity theory is anti-predation. When an organism sleeps, the theory goes, they decrease their risk of being eaten by predators. Sleeping causes an animal to be a small target, hidden away, not making much noise.