The exact function of sleep is not clear.

Many body functions are inhibited when normal sleep is disrupted. These can be from the immune system to memory. People that have poor sleeping habits tend to get sick more often than well rested peers and I am sure that I would not need to remind people on here about the pitfalls of pulling an all-nighter before an afternoon exam the next day.

Food, while it does provide nutrients and energy is not capable of counterbalancing the effect of sleep deprivation but I have no idea why…

...and you don't sleep due to lack of energy; that's mixing linguistic idioms with fact.