[sorry it's so long]

Our son is 4.5 years old and he's been going to sleep by himself since he was about 3 months. It used to take him about 15 minutes, after a couple years it became 20 minutes, and then in the last year it's been more like 30 minutes. However, in the last couple of months it has gotten to be 1 to 1.5 hours. (He's still awake now and has been in bed for 80 minutes.) He's quiet in his room (shares with 20-month-old sister) and we think he's even staying in his bed all of that time. But she's asleep and he's not. Sometimes all his rustling around wakes her up. It's not just tossing and turning. He has stuffed animals and blankets he plays with.

There have been only 2 changes to bedtime. One is that for the last 6 months or so he's been transitioning to no nap. He was napping a couple of times a week and on days he napped, it would take him an hour or so to fall asleep so we just transitioned to no nap at all.

His routine has stayed almost exactly the same except for the inclusion of his sister. For a while we staggered their bed times and he would go to sleep after her (her at 8, him at 9), but we put them to bed together of times for various reasons and he started asking to go to bed at the same time as her. So we moved his bedtime earlier to 8 because I think he still needs the same amount of sleep overall and he's not napping anymore. And he gets up before 8. We'd just put him to bed later but he always tries to get up early because his sister does. (She's usually up at 6:30am.)

So now he doesn't nap and he still takes 1 to 1.5 hours to fall asleep on most nights. He still wakes up before 8 and is cranky and ornery a lot. We give him the opportunity to nap (lie down in the middle of the day for ~20 minutes) in case he wants to but he hasn't for about 2 months. And sometimes he's yawning as we say good night so he's clearly tired, but it just seems like he's staying awake on purpose.

Is there anything else we can do to help him fall asleep? Our evening routines are this: dinner (6pm), bath (7pm), milk and stories, brush teeth, 5 minutes of singing in their room after getting into bed. We try to talk to him about his day so he can process stuff with us instead of all night before going to bed.

His teacher says it's just summer and there's too much daylight (not dark until 10pm - although they have blackout curtains) so none of the kids are getting enough sleep. Is it as simple as that? A seasonal thing?

Could there be a psychological factor? Lately he's been exhibiting more hostility towards his sister and acting more defiant towards us. I don't know if it's the sleep deprivation that's causing crankiness or if he's unhappy and it's making it difficult for him to fall asleep? His teachers say that he's perfectly behaved at summer school. At first, I thought he was just adjusting to school being out, but it hasn't gotten any better.

We can never decide if we should check on him after xx minutes or just let him be. We can never decide how much to let him whisper to his animal friends or hum to himself before going to quiet him down. As it is now, we let him be unless there is excessive thumping sounds, footsteps, or we think he's bothering his sister.

Any advice or opinions or sympathy would be most helpful. If only I could make myself believe that the body can naturally regulate itself and he has the right amount of sleep he needs since we give him enough opportunity to rest.