DS has been really good the last 3 months or so, going to bed no problem and straight to sleep. But he's suddenly started resisting bedtime and once he's asleep, waking, often a couple of hours after going off. Normally he just wakes and wants to play then goes back to bed half an hour or so later - I usually give him another drink of milk and he realises it's time to go back to bed. He only had a hour nap this morning rather than two hours in the afternoon and went off at a reasonable time with no problem. Is it likely that he's starting to need less sleep in the day? He's 16 months.