The rich diversity of sharks extends to more than just their size and shape.
Sharks’ feeding patterns are another thing that makes these animals particularly unusual.
Even when food is in abundance, many sharks will even hunt in shifts, allowing them to stay out of each other’s way.
So, when are sharks most active?
Most sharks are neither nocturnal nor diurnal. It seems that most sharks are more active between the darkness/daylight transitions, at dawn and dusk. This means they’re neither more active in the daytime or the nighttime but are crepuscular.
This leads them to various peaks in activity, covering all times of the day and night. But most sharks that hunt larger prey do so at specific times.
So, when do most sharks hunt? And what are the factors that determine their most active periods?
Are Sharks Nocturnal?
Sharks can be active at all different times. Some are mostly nocturnal; others are more active during the day.
They also have some strange sleeping habits that need to be discussed and which don’t really follow the same rules as terrestrial animals, and this can affect when they choose to hunt.
There are still gaps in our knowledge, but from the summary of what’s been recorded so far, it looks like most sharks forage more and travel farther during the morning and evening hours. This might not fit as well with the narrative of the invisible night-time hunter, but some sharks are more active at night.
Which Sharks are More Active at Night?
Some sharks prefer to hunt at night. This is likely because the much more dangerous sharks are asleep (or a weird version of sleep that happens in the ocean – more on that later!) rather than because their natural food source is more abundant at night, but it can also be related to different hunting strategies.
Here are a few of them:
Most of these sharks will feed on reef prey like crabs, small fish, and octopuses, and they’re typically smaller than some big diurnal predators like tiger sharks and bull sharks.
This would further support the idea that they’re hunting more at night to avoid competition.
Diurnal, Nocturnal, What’s the Difference?
So we’ve mentioned some terms that might need clarification.
Most animals we’re familiar with are diurnal. These are the animals who are awake during the day.
Humans, most birds and reptiles, bees, and butterflies are all examples of diurnal animals, and these are things we see every day.
Then there are the elusive nocturnal animals. Some of the most familiar are owls and bats, as well as moths that come into your house when you leave the light on and the window open (Incidentally, it doesn’t make much sense that a nocturnal animal will get so excited by a bright light, and scientists still aren’t sure why that happens).
Some animals are more active between light and dark, during the early morning and late evenings, and the word for this is so satisfying to say it’s worth repeating: crepuscular. Crepuscular! Sounds good, right?
These animals are most active in dim light, out of the bright daytime sun but taking advantage of the low evening light. They’re often ambush-predators like cats and foxes.
Animals who don’t subscribe to any of their society’s expectations and mess around whenever they feel like it – day or night – have another great name: cathemeral.
One thing to note about crepuscular and diurnal animals is that there has long been a bit of a sampling bias when it comes to spotting them.
Humans have very bad eyesight in the dark, so until the invention of high-quality night-vision cameras, we thought many animals were crepuscular when in fact, they’re nocturnal – we just couldn’t see them at night.
This is an important thing to remember with behavioral research and science in general – it only reflects what we can observe.
So, the period for which animals are most active can be assumed to be the opposite of the period they’re the most sleepy.
So far, all these examples are based on land. And the sea has some interesting new factors to consider. First and foremost: how does an animal sleep underwater?
To understand a bit more about when and why sharks are most active, we have to look at when and how they sleep.
Circadian Rhythm, Sleep Cycles
If you’re a relatively typical human, you’ll wake up in the morning at some time between 6 am, and 10 am, and you’ll get sleepy again before bed at some time between 8 pm and 11 pm.
Of course, there are outliers (like the exhaustingly chipper early-birds or the sullen and mysterious night-owls). But on the whole, humans and other diurnal animals follow a sleep cycle based on the natural light cycles of the day and night.
This is called a circadian rhythm, and it’s basically an internal clock, measurable by recording changes in hormones and brain activity, usually in relation to light levels.
This rhythm determines an animal’s sleep cycle, but when the only place to sleep is a sea bed, you might find it hard to breathe.
A handful of sharks are obligate ram ventilators and need to keep moving to breathe, so how do they sleep?
Ram ventilators swim all the time, but some move slowly at night, suggesting they’re in a more restful state. Since these sharks have to keep moving, they can’t curl up and have a nap in the coral, like some of their more lazy and adorable cousins, but sleep seems to be a biological necessity for all animals, so how do they do it?
It turns out that some sharks have a system of selective sleep.
Dolphins and whales, who need to come up for air regularly, have a system of unihemispheric sleep where the sleep/wake cycle can be partitioned between the two brain hemispheres.
Basically, half their brain is asleep at a time. Sharks do something similar, but it’s thought that they might utilize patterns of intrahemispheric sleep: having different parts of the brain shut down in turns.
These sleep systems allow the shark to maintain motion while resting different parts of its brain. But so far, it’s not clear what the mechanisms are that sharks use to sleep because, in the water, it’s really hard to measure the biochemical changes in the animal’s brain.
Sharks have shown a surprising diversity in circadian rhythms.
While nocturnal sharks such as the horn shark and swell shark spend most of the day sleeping, this is shown in the horn shark to be related to a simple response to the light cycle and not really an internal clock.
On the other hand, the swell shark shows signs of anticipation – increasing its activity moments before the light cycle changes.
There are other factors determining the best time for an animal to sleep. The patterns of their prey, for example, or their predators.
There is almost as much diversity in sharks’ sleeping and activity patterns as in their habitats and species.
But what you really want to know is when are you most likely to get eaten by a shark, right?
What time of day are sharks most likely to attack?
Shark attacks occur by the millions each day. There are uncountable casualties; brutal attacks resulting in immediate death, or botched jobs, dragging out a more slow and painful demise.
They can occur in any sea or ocean, at any time of day or night.
Almost all of these attacks are on crustaceans and fish, but some are on seals or birds, even other sharks. Close to zero attacks are on humans.
Still, humans are occasionally bitten, and there are high-risk areas where it’s probably useful to know when to stay out of the water.
- White sharks are more active in the crepuscular and night-time periods.
- Bull sharks get up early morning to hunt
- Tiger sharks prefer the midday heat
- Sandbar sharks like the afternoons
- Hammerheads prefer the nighttime
So, it’s pretty much a case of ‘pick your poison’. However, most of the larger sharks that can be dangerous to humans are active in the morning and evening.
Tiger sharks are around in the midday, but it’s also been shown that they will hunt whenever they feel like it, perhaps making them the only known cathemeral shark species.
These sharks hunt in shifts because they’re trying not to get in each other’s way.
They have many food preferences in common, so the smaller ones will often take a time out while the bigger ones are eating. In terms of resource partitioning, this is quite an unusual strategy.
As for swimmers, the state of Hawaii recommends staying out of the water at dawn, dusk, and nighttime to avoid being mistaken for food.
Sharks are diverse and prevalent hunters, occupying a significant range of niches across a number of different ecosystems.
Since they are often opportunistic feeders and share their food resources with one another, they’ve learned to adapt to being active at different times of the day to avoid competition.
However, most of the larger hunters are most active during the low-light hours at around dawn or dusk.
Sharks have periods of inactivity where they may sleep in unusual ways. The sharks that need to keep moving rest in short bursts where parts of their brains shut down, and others keep working. This allows them to swim and stay alert while resting important brain regions.