When we get poor sleep, our bodies go into survival mode. This is likely wrought by evolution: Little sleep, in prehistoric days, was probably associated with danger or scarcity.
Scientists now know that sleep deprivation sets off a chain of metabolic responses to ensure our survival: Namely, it makes us hungrier (so we have more energy to face the threat).
In ancient times, this would have helped us stay alive. But in these times of abundant calories, it seems to just propel us to overeat.
