I think you misunderstood, I'm not leaving them to go and sleep.
But you said this earlier:
Then i usually rest them in my hand under my neck or put them in a small box and sleep with this on my chest.
If you sleep with them on your chest in the box you must be sleeping more than 30 minutes right?
 
When they hatch out, the feathers need to dry before they can maintain self heat. After that i believe they are mostly ok. 
That is not right.  The young chicks need a heat source such as a lamp.  If you watch a chick with its mother it will spend some time running and eating, but every so often go back under the mother, and the mothers will sit on it and warm it up. 
Chicks can not regulate their own body temperature.  You would have to keep changing the hot water bottle many times in the day and night - also they need the correct temperature - the water bottle might be too hot or not hot enough.