Help my Son cant sleep longer than 8 - 10 hours a night

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Help, my 4 year old cannot sleep past 6am. This is effecting his concentration in the day as he gets so tired and I fear will hold him back when he starts school. Bed time starts at 7 with a bath and story with his sister (she is just over two and a half). He gets himself off to sleep well but sleeps restlessly and hey presto is up at 6 ish (today & yesterday it was about 4:30)
I don't think it is a case of not needing the sleep as he is just so tired later on in the day.

do you encourage him to go back to sleep or do you get up with him? its very difficult to change the morning wake up time as it seems they have a little internal alarm clock that just goes off, but it can be done by encouraging him to go back to bed

can you pull forward his bed time so that you start bath at 6:45 then 6 :30 then 6pm

have you set your boiler or heating to ocme on at that time? you would be suprprised at hoe many folk's DCs are disturbed by heating clicking on

Also have you considered adding a nap back into his day? My youngest started school still napping, he found school EXHAUSTING for the first year, a whole other story

definitely could be the heating! any morning i want a lie in i turn my heating off, it really works!

The heating isn't high and it set to keep the house at a steady temperature. We do put him back to bed but he doesn't go back to sleep.