I think it has to do with timing and if you've been online already or not. I've noticed that sign in won't show if I've played and time has lapsed into the next day (24:00). Sign in only pops up once during the day and, if you've been on at the time of the new day, I think it still counts. Hence why, when you wake up, you don't see it because you were technically online for the day. It could be something else, but anytime it hasn't popped up for me it's because I was playing and server time reached the next day.