From about 21-22 months K, certainly by 24 months.
Posting with a success story from my friend M. I lent her our Groclock because her 21 month old daughter E had been waking at 4.30am (AFTER lots of NWs) for six months. It was affecting the whole family. M and her husband were taking it in turns to sleep on E's floor and called it 'the room of doom'. E's older brother had to move out of the shared bedroom and go in with his parents. E was having huge tantrums, resulting in her head butting herself against the cot and really bruising up her little head. Nobody was having a good time.
I sent the Groclock at the beginning of Feb, M made really sure that E understood what the moon meant and what the sun meant, they role-played what you do if you wake up and see the moon (I no longer had the little book that comes with the clock so they had to manage without it). We agreed that E could sleep without mum or dad on the floor as she was going to sleep independently at bedtime and for her nap. The very first morning she seemed to get it. She moaned and fussed and cried a bit but without the fire and fury of her normal morning tantrums. She did lie down - popped up and down for half an hour or so before the sun came on. Same the second morning - lying down, and less crying. The third
night was a bit of a mess with NWs (she was teething and had a cold) BUT she did go back to sleep when she EWd, and had to be woken up at 7am. By the end of the first week she was sleeping until 7-7.30am every day and continues to do so! Mum and Dad are still enjoying the beauty of unbroken nights and a 7am start, big brother is back in his own bed - harmony is restored!
