My 18 mo son has always had strange naps. He cries after 30 mins, then if he goes back to sleep wakes again after another 30 mins crying and then it's either game over or he manages to do a little bit more. What I don't understand is why. He has just had an hours nap just now and is now standing up in his cot crying as still tired. He connects sleeps cycles at night time easily and he gets himself to sleep for naps and bedtime so I don't underhanded why he has always had nap issues. Any ideas?