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We used to be able to put him to bed give him a kiss and he would go to sleep.
Has anyone else experienced this kind of transformation?
Yes but when DD was a tad older - around 2y6-8m. She went from happily going to bed and to sleep at 8.30pm (even after napping until after 4pm) to regularly reappearing or coming to her door for up to an hour or more after bed time (eg 10pm).
We ditched her nap and moved bedtime back to 7pm and she went back to no fuss bedtimes. The main hassle doing this was having to give her super-early dinners before she got too tired to eat. If she fell asleep in the car in the afternoon some days she would sleep from 4pm until the next morning.
If your DS is having late or long naps can you make them earlier or shorter? For a while I did manage to get DD to nap earlier (ie lunch at 11 then asleep by 12) but mostly she was not tired enough to go to sleep until 2pm and then would sleep until 5pm. Waking her from naps has pretty much always resulted in grumpy tanties for over 1/2h

so I avoided wherever possible.
Edited to add that we have never ever told DD she has to go to sleep, just that it's bedtime and she needs to stay in her bed. The light is off and the door is shut so provided she is quiet I don't care if she lies awake for 1/2h. I just tell her to relax and shut her eyes and she will drift off to sleep - and she gets the same line if she wakes at night from a bad dream.
Steph