What causes people to wake up irregularly in the night?

Sleep apnea is a common cause. You can't breathe, so your brain wakes you up to correct the problem.

Alcohol can be a cause. It depresses the brain and puts you to sleep, but when it wears off it has messed up your normal sleep cycle.

Urinary blockage can be a cause, especially in older men. They can't empty their bladders before going to bed, so they have to get up in the middle of the night.

General stress and anxiety can wake you up, too. You brain is thinking about stuff all night, so you wake up to worry about it some more.