Which problem is that
Answer it in your head first, then look.
Someone gets into bed at eleven, lies there completely alert until half past three, then sleeps solidly until eleven in the morning if nothing wakes them. Which of the four is that closest to?
Late sleep, which usually points to a body clock set later than the working world rather than to insomnia. The giveaway is that the sleep itself is fine once it arrives. That pattern has a different treatment from the others, and lesson six says so, because sleep tips will do almost nothing for it.