Going to bed; retiring for the night (a relatively formal/official term).
毎晩十一時に就寝しています。
I go to bed at 11 every night.
病院では就寝時間が決まっている。
At the hospital, there is a set bedtime.
子どもを早めに就寝させた方が翌朝機嫌がいい。
If you put children to bed earlier, they are in a better mood the next morning.