Nocturnal describes an animal that does most of its feeding and moving at night and rests during daylight. Among sharks it applies to a long list of reef and coastal species: nurse sharks, whitetip reef sharks, horn sharks, swellsharks, catsharks and the epaulette sharks all spend the day wedged under a ledge and the night working the bottom.
The pattern is about opportunity rather than any dislike of light. Prey that hides in crevices by day comes out after dark, and a slow shark with a short strike does far better against it then. Sharks that hunt by chasing, such as the makos and the blue shark, gain nothing from the switch and keep no fixed schedule.
- OppositeDiurnal — active by day
- Dawn and duskCrepuscular
- Night vision aidThe tapetum lucidum
On this page
What the word means, and its neighbours
Nocturnal means principally active at night. Its opposite is diurnal, and between the two sits crepuscular, for animals that concentrate their activity into the short windows around dawn and dusk. A fourth term, cathemeral, covers species with no consistent pattern at all, which describes a surprising number of large sharks once they are tracked properly.
The distinction is behavioural, not anatomical. Nothing about a shark's body forces it into one schedule, and several species shift with the season, the tide or the availability of food. Reef sharks in areas with heavy daytime boat traffic have been recorded moving their activity later; the same species elsewhere hunts freely in the afternoon. Treat the label as a description of what an animal usually does, not a rule it is bound by.
It also has to be earned by evidence. For most of the twentieth century a species was called nocturnal because divers saw it resting during the day, which says more about when people are in the water than about the shark. Acoustic tags and accelerometers have since confirmed some of those labels and overturned others, and a few species long described as night hunters turn out to move steadily around the clock.
Which sharks work at night
The nurse shark is the standard example. Through the day it lies motionless under coral ledges and in caves, often piled in groups of several animals, pumping water over its gills and moving for nothing. After dark it noses along the reef base, using its barbels to find crabs, lobsters, urchins and sleeping fish, and taking them with a powerful suction bite that pulls prey straight out of a crevice.
The whitetip reef shark does something more coordinated. Groups that spent the day resting in the same cave emerge together and work a section of reef as a loose pack, forcing their slender bodies into holes after fish that have shut down for the night. The commotion attracts more whitetips, and a single hidden fish can end up with several sharks jammed into the same crevice.
Further down the size range, the horn shark leaves a fixed daytime crevice at nightfall, forages over a small home range and returns to the same spot before dawn. The swellshark follows the same routine off California. The epaulette shark is the most extreme: it works the reef flat at night on a falling tide, crossing exposed rock between pools on its paired fins and tolerating oxygen levels that would kill most fish.
The equipment that makes it work
The key structure is the tapetum lucidum, a mirror of guanine crystal plates sitting behind the retina. Light that passes through the retina without being absorbed hits the plates and is reflected straight back through it, giving the photoreceptors a second chance at every photon. It is the same device that makes a cat's eyes glow in a torch beam, and in sharks it is unusually well developed. Several species can slide a screen of pigment over the mirror in bright conditions so the advantage does not become a liability by day.
Retinas help too. Shark eyes are dominated by rods, the cells that work in low light, and the studies that have looked for colour vision in sharks have mostly failed to find it — several species appear to have a single cone type or none, which trades colour for sensitivity. The page on whether sharks are blind covers what that actually means for their sight.
Beyond the eye, none of a shark's other senses care whether the sun is up. Smell, the lateral line and the ampullae of Lorenzini work identically at midnight, and the last of those takes over entirely in the final moment of a strike. A shark hunting in the dark is not handicapped; it is using most of the same equipment it uses at noon.
Where the word stops being useful
Below a few hundred metres there is no night, because there is no day. Deep-sea sharks live in permanent darkness and cannot be nocturnal in any meaningful sense. What they often do have instead is a vertical rhythm: the cookiecutter shark rises towards the surface after sunset and drops back at dawn, following the layer of small fish and squid that makes the same journey. The clock is the same, but the axis is depth rather than activity.
The other limit is the assumption that night is dangerous water. Swimming at dusk, dawn or after dark is discouraged in most safety guidance, and the reasoning is sound — visibility is poor in both directions and the species that feed close inshore are more active. But the sharks that account for serious incidents are not nocturnal animals, and the caution is about conditions rather than about a schedule. The shark attacks section sets out what the records actually contain.
For the rest of the terms used across these pages, see the shark terms glossary.
Frequently asked questions
What does nocturnal mean?
It describes an animal that does most of its feeding and moving at night and rests during daylight. The opposite is diurnal. Animals concentrated around dawn and dusk are called crepuscular, and those with no fixed pattern are cathemeral, which fits a good many large sharks once they are tracked.
Which sharks are nocturnal?
Nurse sharks, whitetip reef sharks, horn sharks, swellsharks, epaulette sharks, Port Jackson sharks and most catsharks. All of them are relatively slow, bottom-oriented hunters that rest in caves or crevices through the day and come out after dark to work the sea floor for crustaceans, molluscs and sleeping fish.
How do sharks see at night?
They have a tapetum lucidum, a layer of reflective guanine plates behind the retina that bounces light back through the photoreceptors so each one gets a second chance at it. Shark retinas are also rod-dominated, which favours sensitivity over colour. Some species can cover the mirror with pigment in bright light.
Are deep-sea sharks nocturnal?
No, because there is no day or night where they live. Below a few hundred metres the water is permanently dark. Many deep-water species do follow a daily cycle of a different kind, rising towards the surface at dusk and descending at dawn behind the small fish and squid they feed on.
Is it more dangerous to swim at night?
Safety guidance discourages swimming at dusk, dawn and after dark, mainly because visibility is poor for the swimmer and for the shark, and inshore species feed more actively then. The species involved in serious incidents are not themselves nocturnal, so the advice concerns conditions rather than any night-hunting habit.
References
A small team focused on researching sharks and making marine biology easy to understand. Every page is reviewed against current species records and the SharkSider attack database, and updated when new incidents or research appear.
Published December 2, 2014 · Last updated September 23, 2026 · Part of the Shark Facts section.
