Mum had my long hair untied for nights and whinged about the knots in the morning.

When I was older I heard about tying hair back for sleep. I did it then and whenever my hair is long enough to pull back I do it now!

Depends on the child's hair. Fine hair, no knots, don't bother. Thick hair, knots, just tie it back (very loose braid is good) and make life easier on all of you.