Quiet as a mouse – why and how the hazel dormouse hibernates
7th Jan 2026
At this time of year, the cute little hazel dormouse, vulnerable to extinction in the UK and captive-bred at Wildwood for UK-wide reintroduction, will be deep asleep in its tightly woven nest of shredded bark, leaves and moss, having almost doubled its weight in preparation for a hibernation that in the wild can last from October to April.
Sleep as a survival strategy
The dormouse, whose name is thought to derive from the French “dormir” - to sleep, is also known as “sleep mouse” and “dozy mouse”, because it not only hibernates in winter but goes into torpor in summer.
This is because it has a very fine fur that gets waterlogged easily, which coupled with its small size (6-9cm plus tail), means it loses heat more quickly than other small mammals.
So, to avoid foraging in wet and cold, or if there’s no food around, it uses torpor to save energy in summer, sleeping for up to 24 hours, then waking up to see how things are. In winter it hibernates for up to seven months.
Preparing for the big sleep
The trigger for going into hibernation is a combination of day length and the first frosty weather. Like bears, dormice pile on weight before their winter hibernation.
Unable to digest grass and other vegetation, they have a particular diet that requires a diverse habitat - one reason for their population decline.
As tree- and hedgerow dwellers, in spring they eat soft tree flowers, pollen and nectar, in summer insects, and in autumn fruits and nuts, building up their brown fat store for hibernation.
To find this food in succession through the year is not easy. Oak, hazel, bramble and honeysuckle are the four most important trees for dormice.
Temperature and moisture regulation
Hibernation is the only time dormice come down to the ground.
Although this makes them more vulnerable to predators, they need a stable temperature and humidity for hibernation, when their body temperature drops to the outside temperature and when they risk desiccating if they lose too much moisture.
To preserve moisture, they build a tightly woven nest in a depression in the ground or a hollow where dew and rainfall collect, and sleep curled up in a ball with their tail over their nose.
In hibernation, the dormouse drops its heart rate to just a few beats and its breathing to just a few breaths per minute.
Mysteries of dormouse hibernation
There’s much we still don’t understand about dormouse hibernation.
For example, every 10 days or so, they do warm themselves up.
This could be to remove toxins or to boost their immune system or another reason.
Generally, they don’t leave the nest at this time, but some have been seen moving, for reasons we also don’t yet understand.
And if they do need to wake up from hibernation quickly, they do a kind of snoring, possibly to get oxygen into their body.
Summer is for breeding and young
In summer, dormice nest up a tree or in dense bramble or hedgerows.
The nest is looser woven than in winter as they move about and have young.
They typically have one to two litters with an average of four to five young, but could have up to eight.
Dormice generally have their first young in June, but can have a litter as late as September or October.
As the young are with them for two to three months, these later born animals may not build enough fat for hibernation, but our recent Indian summers and mild winters increase their chance of survival.
Indeed, due to the weather, 2025 was a bumper year for dormice at Wildwood with one pair managing three litters!
Our work with dormice
Wildwood’s dedicated conservation team are leading experts in breeding dormice for the national reintroduction programme, overseen by People's Trust for Endangered Species, which has seen over 1000 dormice successfully reintroduced to suitable woodlands where they have previously become extinct.
As studbook keepers for the species, conservationists at Wildwood liaise with the national Common Dormouse Captive Breeders Group to select and pair up the animals, ensuring a healthy and strong genetic mix for future reintroductions.
