If people you know tweet enough, I suppose a little examination is all you need to really figure out when they're sleeping. But if not, you can always use Sleepingtime.org, a site dedicated to telling you when, exactly, any Twitter user is most likely to be sleeping. Just enter someone's username into the search bar, click "Get Sleeping Schedule," and the site will tell you when they are most likely to be sleeping. It also tells you which time zone they're located in and how many past tweets were analyzed to come up with the sleeping estimate. I'm sure this site was made just for fun, and it's not like it uses information that's not public already, so what do you think?