Once males leave the nest they do not go back, so they have to find somewhere to spend the night. Hanging underneath the heads of flowers or even getting right into them is what they normally do. On the right you can see a photograph of a Bombus hortorum male deep inside a flower. The photograph was taken in the morning before the sun had warmed up the garden and the bee could barely move. Below right is male hanging underneath the head of a teasel.
Their temperature will drop and by morning they will have used up their stores of energy, so until they warm up by either drinking nectar or sitting in the sun or both, they will appear listless, sick, and are unable to fly.