What Do Whales Do at Night?

Many whales and dolphins sleep during the night. Whales, unlike humans who are involuntary breathers, must decide to take each breath. Accordingly, whales cannot spend long periods underwater without surfacing. Dolphins -- which are part of the same family that some whales are -- sleep while swimming, but they have a unique adaptation: only one side of the dolphin brain sleeps at a time. When a dolphin requires a breath, the active half of the brain signals the dolphin to surface. Scientists have also documented gray whales (Eschrictius robustus) sleeping with one eye open. Some whales sleep while drifting horizontally -- a behavior termed “logging” -- while others rest on the bottom. Sperm whales (Physeter macrocephalus) sleep while hanging vertically near the surface of the water.

Scientists have collected data that suggest some whales are not just sleeping at night but are dreaming, too. Publishing their results in a 2001 issue of “Journal of Sleep Research,” researchers with the Russian Academy of Sciences of Moscow, Russia and the University of California in Los Angeles studied the sleeping habits of a rescued gray whale in a captive facility. The researchers found that the whale would sleep in a number of ways and in various states of consciousness. Sometimes, when resting on the bottom, the whales would exhibit sudden twitches or contractions. The scientists noted eyelid movements during 40 percent of the twitching episodes. The next year, a different group of Russian and American scientists, this time representing the Severtsov Institute of Ecology and Evolution in Moscow, and UCLA’s department of Psychiatry, documented similar behavior in beluga whales (Delphinapterus leucas).

Killer whales (Orcinus orca) occur in both resident fish-eating populations and transient pods that consume sea mammals. The mammal-eating groups are vocal, calling extensively when hunting and eating. In contrast to previous studies, in which scientists searched for feeding activity visually, researchers used acoustic recording devices in their 2008 study area. Publishing their results in “Marine Mammal Science,” researchers Newman and Springer documented the killer whales communicating extensively at night -- a previously undocumented phenomenon.

Scientists presume that the black color of short-finned pilot whales (Globicephala macrorhynchus) is an adaptation related to their nocturnal hunting habits. Short-finned pilot whales make very deep dives, sometimes travelling almost 2,000 feet below the surface. This timing corresponds with the major movements of bottom dwelling organisms, and the activity peaks when light levels change, around dawn and dusk. Pilot whales consume up to 100 pounds of food each night, primarily composed of different squid species.