I know when I've had these really down instances, even though I was physically capable of getting out of bed, staying there curled up by myself felt the most comforting thing to me at the moment. After a while I got up but for a bit I didn't feel like leaving the confines of my bed. This was more so with me in that I wasn't unable to get out of bed, just unwilling for that time. Sometimes was during college when I didn't really have to and other times after I started working, I was able to go in late or use sick leave so I was fortunate.