Not only do dormice hibernate for around half the year, they also spend a large part of early summer in a state called ‘torpor’. Torpor is somewhere between sleep and hibernation, and helps them save energy on cooler days. They also sleep through most of the day, even at the height of summer.
NB. for pedants - hibernation is different from sleep, as is torpor. But it's not such as punchy fact if I try to explain that in the headline.