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 is male hanging underneath the head of a teasel.
Their temperatures 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 and sick.
I get a huge number of emails from people asking me why their bees are sick, when in fact they are just males who have spent the day chasing queens and drinking nectar and then stayed out all night. Sometimes it rains and they get soaking wet, but they will recover once they drink or get warmed up by the sun.
Sleeping inside a disk or bowl shaped flower is a good strategy for these bumblebees and other insects as research has shown that the temperature at the base of the bowl, near the source of nectar, can be as much as 10 °C higher than the surrounding air temperature (see the bumblebees inside the courgette flower on the behaviour 2 page).