How do Whales and Dolphins Sleep?

This is one of the more common questions that passengers ask while on safaris with us, and the answer might surprise you!

The first thing to consider about whales and dolphins is that they are conscious breathers. This means that they must choose to come to the surface and take each breath. They do not have a breathing reflex which takes over during periods of unconsciousness or sleep like humans do, so they must actually remain conscious even during rest, or they risk suffocating or drowning.

To get around this, dolphins and whales actually have a few really amazing adaptations which allow them to rest periodically throughout the day or night as conditions and behaviours allow. They enter a state of sleep called ‘unihemispheric slow wave sleep’ or USWS. Essentially, one hemisphere of the brain (and the opposite eye!) sleeps at a time while the other remains in an active state, in order to watch for predators or obstacles, to react to danger, and to signal when to rise to the surface for a breath of fresh air. These periods are taken opportunistically, and it is possible for the animal to move quickly from a state of USWS to full wakefulness if, for example, a predator presents itself.

If undisturbed, it has been shown that dolphins generally prefer to spend about 2 hours for each side to rest at a time, but a few periods like this can be taken during the day or through the night. For example, with the use of EEG (brain activity) readings in bottlenose dolphins, one study found that they spend on average about a third of their day sleeping in this way.

There are a few other behaviours and adaptations which allow this strategy to work quite effectively. First, as dolphins are highly social animals, they have the advantage of safety in numbers. Adult dolphins have a tendency to pair up and swim side-by-side with their companion while resting. Additionally, while some individuals in the pod might be resting and moving quite slowly, others of the group might circle a bit wider, and can alert others to wake if they sense danger, obstacles, or a good opportunity to feed. Dolphins also slow their breathing down considerably when they are sleeping, allowing them to recover energy from rest more efficiently. The number of times that they need to come to the surface for breaths goes from 8-12 per minute while awake down to 3-7 per minute while sleeping.

It has also been shown that species which use USWS do not have a disadvantage in terms of health or brain activity. Dolphins actually compensate for the lack of complete sleep due to their efficient immune systems, brain plasticity, ability to thermoregulate (keep their internal body temperature stable), and their brain energy metabolism. All of these things would be significantly compromised for us humans if we did not have complete sleep!

Resting in such a dynamic and unpredictable environment as the ocean presents its challenges, but the mechanisms for whales and dolphins to rest effectively have evolved to meet their needs while keeping them safe.

References:

• Rasul, N. and Stewart, I. (2019). Oceanographic and Biological Aspects of the Red Sea. 1st ed.  (pp. 305-326). Springer, Cham.
• Mascetti, G. G. (2016). Unihemispheric sleep and asymmetrical sleep: behavioral, neurophysiological, and functional perspectives. Nature and Science of Sleep, 8, 221.
• Sekiguchi, Y., Arai, K., & Kohshima, S. (2006). Sleep in continuously active dolphins. Nature, 441(7096), E9-E10.