That's usually when I realise I need to sleep. My interoception is really bad, so I notice when I'm feeling tired. I will notice when I start getting general anxiety and depressive feelings, and those always* go away with sleep.
Learning how to relax, taking walks in nature, and spending time with friends have also worked for me. Deciding which one will help at any given time is an art I have yet to master.
*when no external factors are present.