Elevated stress levels among Norway's youngest children in childcare
Norwegian researchers measured the stress hormone cortisol in 112 toddlers from 85 different childcare centers in six municipalities, approximately five months after they started attending. Children with the longest childcare days (8-9 hours) showed increases in cortisol during the day. Ref. Source 2q.
Well you think about it. If you are away from your parents and with strangers it can add to the stress of a young child. The children always seem much happier and less stressed when they are with their parents. I am sure that all changes when they are tens and they are then less stressed when away from their parents.