premise: I do n't know why , but I suspect it 's because at night it 's dark , and he has become used to falling back asleep , like we all do at night . During the day when it 's light , he wakes up and ca n't put himself back to sleep yet . The other bad thing is that until now , I ' ve let him take his afternoon nap in the swing , which is becoming a problem because I think he is fast approaching the weight limit ( I ' m fairly certain the limit is 20 lbs , and he was 16 about three weeks ago ) . So pretty soon , there will be no swing to nap in . What 's a possible reason he ca n't put himself back to sleep during the day ?
hypothesis: Because it 's too bright .