In this task, you're given a context passage. Your job is to generate relevant questions that can be answered by directly referring to the passage.
Q: I don'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 can'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 .
A:
What's a possible reason he can't put himself back to sleep during the day ?