Most squirrels you see in parks and backyards are strictly daytime animals that spend the night asleep in nests or tree cavities. Fox squirrels, gray squirrels, red squirrels, and ground squirrels are all diurnal, meaning they are active during daylight hours and retire to shelter well before dark. The major exception is the flying squirrel, which flips the schedule entirely and does virtually all of its foraging, traveling, and socializing after sunset. What any given squirrel does at night, then, depends almost entirely on which kind of squirrel it is.
Tree Squirrels Shut Down After Dark
If you have ever wondered why squirrel activity in your yard seems to vanish as dusk approaches, there is a straightforward physiological reason. Tree squirrels like fox squirrels and eastern gray squirrels have strong 24-hour internal clocks that keep them on a tight day-active schedule. A study using temperature-logging collars on fox squirrels found that every animal showed a robust daily rhythm: body temperature stayed consistently high at night while the squirrels remained curled up inside their nests, and fluctuated during the day as they moved in and out to forage.
1Frontiers in Ecology and Evolution. Individual Daily and Seasonal Activity Patterns in Fox Squirrels (Sciurus niger) Quantified by Temperature-Sensitive Data LoggersIn other words, tree squirrels at night are doing almost nothing. They are tucked away, asleep or resting, conserving energy until morning. Their eyes are built for daytime vision too. Unlike most mammals, tree squirrels have cone-dominated retinas suited for bright, color-rich environments rather than the rod-dominated retinas that help other animals see in the dark.
2Current Biology. Shortwave visual sensitivity in tree and flying squirrels reflects changes in lifestyleWhere Tree Squirrels Sleep
Tree squirrels use two main types of sleeping sites: tree cavities and dreys. A drey is the ball-shaped leaf nest you often see wedged in a fork of branches high up in a tree. These can look messy from the ground, but they are layered structures with an outer shell of twigs and leaves and an inner lining of softer material. Tree cavities, on the other hand, are holes in trunks or large branches, often old woodpecker holes that have been enlarged over time. Which type a squirrel prefers varies by species. In one study comparing Abert’s squirrels and red squirrels in the same area, Abert’s squirrels used dreys most of the time, while red squirrels favored cavity nests.
3Journal of Mammalogy. Potential for Nest Site Competition between Native and Exotic Tree SquirrelsThe material squirrels choose for lining their nests is surprisingly deliberate. In a study of northern flying squirrels and red squirrels in Ontario, researchers found that the vast majority of nests from both species were built almost entirely of shredded bark from eastern white cedar. The average nest depth was about 12 centimeters, and there was no real difference in nest dimensions between the two species or between spring and summer nests.
4The Canadian Field-Naturalist. Cavity Nest Materials of Northern Flying Squirrels, Glaucomys sabrinus, and North American Red Squirrels, Tamiasciurus hudsonicus, in a Secondary Hardwood Forest of Southern OntarioInsulation quality matters, especially in cold climates. European ground squirrels, for example, strongly prefer fresh fescue grass for nest building over dry grass. Lab experiments showed that fresh grass nests actually insulate better than dry grass nests, apparently because the pliable, moist blades pack together more tightly and create a denser structure.
5Journal of Mammalogy. Nest material selection affects nest insulation quality for the European ground squirrel (Spermophilus citellus)Ground Squirrels Sleep Underground
Not all squirrels sleep in trees. Ground squirrels, as the name suggests, sleep in underground burrow systems. These are not simple holes. Richardson’s ground squirrels, for instance, spend roughly 84% of their entire lives underground, either sleeping at night during the active season or hibernating during winter. Females tracked with radio collars used six to eleven different underground chambers over a 15-week active season, shifting sleeping spots regularly. During the first half of the active season, which covers mating, pregnancy, and nursing, females typically rotated through five or six chambers, often staying in one spot for several consecutive nights before moving on. For the much longer 37-week hibernation season, they settled into a single site.
6Oxford Academic (Journal of Mammalogy). Seasonal Use of Subterranean Sleep and Hibernation Sites by Adult Female Richardson’s Ground SquirrelsThat habit of switching chambers is likely driven partly by parasite management. In a study of social African ground squirrels, females whose parasites were experimentally removed raised nearly four times as many offspring to emergence as untreated females.
7Oxford Academic (Behavioral Ecology). Parasite removal increases reproductive success in a social African ground squirrelRotating sleeping sites may help squirrels break the buildup cycle of fleas and other nest parasites, though predator avoidance and food access also play roles in when and why they move.
Flying Squirrels Own the Night
Flying squirrels are the rebels of the squirrel family. While their closest relatives are strictly daytime animals, all flying squirrel species are nocturnal or crepuscular, meaning they are active at night or around dawn and dusk. They glide between trees using a membrane of skin stretched between their front and back legs, foraging for nuts, seeds, fungi, lichens, and occasionally insects or bird eggs in total darkness.
Their shift to nighttime activity has reshaped their biology in fundamental ways. Flying squirrels have rod-dominated retinas suited for low light, the opposite of their tree squirrel relatives. And they have lost color vision entirely. Genetic analysis of both Siberian flying squirrels and northern flying squirrels from North America confirmed that their shortwave-sensitive visual pigment gene has been knocked out by deletion mutations. They simply do not need color vision in the dark.
2Current Biology. Shortwave visual sensitivity in tree and flying squirrels reflects changes in lifestyleDuring the night, Siberian flying squirrels strongly prefer to travel through mature forest rather than across open or urbanized areas. When urban habitat did fall along their movement paths, they moved faster and covered longer distances, apparently trying to cross exposed terrain quickly. Males in heavily urbanized home ranges actually traveled shorter total nightly distances, possibly because navigating fragmented habitat is costlier and riskier.
8PubMed Central. Home-range use patterns and movements of the Siberian flying squirrel in urban forests: Effects of habitat composition and connectivityDuring the day, flying squirrels sleep in tree cavities, nest boxes, or dreys. Radio-tracked Siberian flying squirrels in managed forests in Finland used multiple nesting sites for daytime roosting, including old woodpecker cavities and dreys.
9Wildlife Biology. Home ranges and habitat use in the declining flying squirrel Pteromys volans in managed forestsSouthern flying squirrels tend to nest in cavities of large deciduous trees, while northern flying squirrels use a wider mix of cavities, dreys, and even subterranean nests. Both species prefer larger-diameter trees, especially in winter.
10Wildlife Research. Seasonal nest use of sympatric North American flying squirrelsHuddling Together to Stay Warm
One of the more striking nighttime behaviors, particularly among flying squirrels, is communal nesting. Southern flying squirrels form large aggregations inside nest-lined tree cavities during winter, packing multiple individuals into a single shelter to conserve body heat.
11Journal of Mammalogy. Seasonal Thermogenesis of Southern Flying Squirrels (Glaucomys Volans)The energy savings from huddling are substantial. At freezing temperatures, southern flying squirrels huddling in groups of three saved roughly 20 to 37% of the energy they would have burned alone. Groups of six saved even more at moderate cold, with reductions of about 26 to 36% in daily winter energy use.
12Canadian Journal of Zoology. Winter energy expenditure and the distribution of southern flying squirrelsAt the coldest temperatures tested (around minus five degrees Celsius), the benefit of huddling was clear, but interestingly, there was no significant additional energy saving from being in a group of six versus a group of three. Huddling matters a lot; more huddling partners beyond a certain point matters less.
13Southeastern Naturalist. Metabolic Benefits of Huddling during Acute Thermal Stress in Southern Flying Squirrels (Glaucomys volans)Tree squirrels huddle too, though less is known about how large their winter aggregations get. Gray squirrels will share a drey or cavity with a few other individuals on cold nights, and red squirrel mothers remain in the nest with their young for long stretches during winter, timing their brief foraging trips to the warmest part of the day to minimize heat loss for both themselves and their litters.
14Oxford Academic (Journal of Mammalogy). Nest attendance of lactating red squirrels (Tamiasciurus hudsonicus): influences of biological and environmental correlatesPredators That Hunt Sleeping and Active Squirrels
The division between daytime and nighttime squirrels creates a split in what hunts them. Day-active red squirrels face goshawks and other diurnal raptors. Night-active flying squirrels face owls. A study in Finland found that Ural owl presence strongly reduced flying squirrel occupancy of nest boxes, while goshawk presence reduced red squirrel occupancy. Each predator’s hunting schedule matched the activity schedule of the squirrel species it affected most.
15PLOS ONE. Predation risk landscape modifies flying and red squirrel nest site occupancy independently of habitat amountThe owl effect was not just a one-time finding. Long-term monitoring of Siberian flying squirrels over nearly two decades confirmed that the Ural owl index was negatively associated with nest-box occupancy in both summer and winter, particularly for resident squirrels that stayed in one area year-round.
16Scientific Reports. Weather and biotic interactions as determinants of seasonal shifts in abundance measured through nest-box occupancy in the Siberian flying squirrelSmall mammalian predators are another threat, particularly weasels. In a canopy predator-prey study, weasels were by far the most effective predators encountered, causing deaths in about 10% of their detections near arboreal nests, outpacing owls and other predators by a wide margin.
17PubMed Central. Predator-prey interactions in the canopyWeasels can follow squirrels right into their tree cavities or dreys, which makes nest-site selection and regular nest switching a matter of survival, not just comfort.
What Happens During Hibernation
Ground squirrels that hibernate present a fascinating case of what “sleeping at night” even means. During hibernation, body temperature plummets and metabolism drops to a fraction of its active-season rate. But the internal clock does not stop. Golden-mantled ground squirrels in deep hibernation still show circadian body-temperature rhythms, just with much smaller swings. Their periodic arousals from hibernation, during which body temperature shoots back up to normal for several hours, happen at a predictable phase of that rhythm. Researchers found they could predict the timing of arousal to within an hour of any 24-hour period once the rhythm’s period was established.
18PubMed. Persistence of circadian rhythmicity in hibernating ground squirrelsThose periodic arousals serve a purpose that researchers initially found puzzling: the squirrels apparently need to warm up in order to sleep. Arctic ground squirrels consistently showed normal sleep patterns during their brief arousals from torpor, including the characteristic decline in slow-wave brain activity that signals sleep debt being paid off. The working hypothesis is that actual restorative sleep cannot happen at the extremely low body temperatures of deep torpor, so the animals periodically return to normal temperature specifically to catch up on sleep.
19Neuroscience Letters. Warming up for sleep? — Ground squirrels sleep during arousals from hibernationWhen ground squirrels do sleep during hibernation arousals, it is exclusively non-REM sleep, and only when body temperature rises above about 10 degrees Celsius. REM sleep, the stage associated with dreaming in humans, appears to be absent during torpor entirely.
20PubMed Central. The Relationship of Sleep with Temperature and Metabolic Rate in a Hibernating Primate: A Study of Sleep in a Hibernating PrimateNighttime Calls You Cannot Hear
Flying squirrels are vocal animals at night, and much of what they say is inaudible to humans. North American flying squirrels produce multiple call types that extend well into the ultrasonic range, above the ceiling of human hearing. Humboldt’s flying squirrels make at least four main call types: arc chirps, tonal chirps, upsweeps, and trills. The timing and rate of these calls shift with the seasons. Trills, the most complex of the four, were produced at higher rates in summer, potentially coinciding with breeding season.
21PubMed Central. Seasonal variation in the ultrasonic vocal activity of Humboldt’s flying squirrel (Glaucomys oregonensis)This ultrasonic tendency is not random. Across the entire squirrel family, nocturnal species produce calls with significantly higher frequencies than diurnal species, even after accounting for body size. This pattern is striking enough that some researchers have floated the idea that flying squirrels might use high-frequency calls for something beyond communication, possibly echo-based navigation while gliding through dark forests, though direct evidence of echolocation has not been found yet.
22Frontiers in Ecology and Evolution. Think Before They Squeak: Vocalizations of the Squirrel FamilyThe Pink Glow of Flying Squirrels
In 2019, researchers reported something unexpected: all three species of New World flying squirrels glow vivid pink under ultraviolet light. The fluorescence showed up on both the belly and back, in males and females, across their entire geographic range from Central America to Canada, and even in museum specimens up to 130 years old. None of the diurnal squirrel species tested, including eastern gray squirrels, fox squirrels, and red squirrels, showed any fluorescence at all.
23Journal of Mammalogy. Ultraviolet fluorescence discovered in New World flying squirrels (Glaucomys)Nobody knows what the pink glow is for. Hypotheses range from communication between flying squirrels (who might perceive UV-excited fluorescence differently than we do) to mimicry of owls, some of which also fluoresce under UV light. It could also be a functionless byproduct of fur chemistry. What makes it interesting in the context of nighttime behavior is that it is exclusively a feature of the nocturnal squirrel lineage. Whatever it does, it does it in the dark.