It can be so many things, different for each person and depending on the day. Sometimes if you look at screens too soon before bed, it makes it harder for your body to produce the chemicals it needs to quiet down. Sometimes you're just wired from conversations and things that happened recently. Intentional, aware breathing can help that. It can also be from lack of exercise. I've also noticed when I'm less productive during the day, I think my body feels like it has to stay awake bc I didn't get much done, lol!