There are over 4,500 migratory species on Earth. Some of them travel such long distances that you might think they experience multiple episodes of jet lag.  

For instance, the Arctic tern (Sterna paradisaea) is the record-holder for the longest roundtrip distance by air (44,100 miles), the Caribou travel the longest roundtrip distance on land (745 miles), and the eastern gray whale (Eschrichtius robustus) travels the longest in water (up to 14,000 miles). 

However, you’d be surprised to know that none of these migratory animals get jet-lagged. On the other hand, humans often experience jet lag when they fly between different time zones. 

“Animals are also very sensitive to day length, but are unlikely to get jet-lagged,” Charalambos Kyriacou, professor of Behavioural Genetics at the University of Leicester, said.

Slow and gradual migration benefits animals

Before we delve into why animals don’t experience jet lag, it is important to understand why humans do.

When a person enters a new time zone, they may experience daytime fatigue, disrupted sleep, digestive issues, mood swings, etc for a few days. This happens because their biological clock (also called the circadian rhythm or sleep-wake cycle) is not in sync with their usual time zone.  

The primary cause of jet lag is the sudden shift in the light-dark cycle, which affects the body’s production of melatonin, a hormone that regulates the sleep-wake cycle. 

The production of melatonin is sensitive to light. For instance, during the day, the light in your surroundings inhibits melatonin production, keeping you awake. At night, darkness stimulates melatonin production, promoting sleep.

What’s interesting is that most animals including migratory birds, fish, and mammals also have melatonin. However, the difference is that the change in time zone or light conditions for most animals is gradual whereas for humans it is sudden. 

Humans travel using airplanes, allowing them to travel from one corner of the world to another in a couple of hours. Their bodies don’t get enough time to adjust to the sudden change in light settings. Their sleep-wake cycle is yet to realize the change in the day-night cycle.

For birds and other migratory animals, the movement from one time zone to another is slow often taking weeks or months. This gives their bodies plenty of time to adjust their melatonin production as per the changing photoperiod (duration of daylight in 24 hours). 

Evolution also saves migratory animals from jet lag

The difference in the way humans and animals migrate is not the only reason the latter don’t feel jet-lagged. There are some more factors at play and evolution is one of them. 

Over time, migratory animals developed several behavioral and physiological adaptations that work as effective migration strategies. For instance, salmon undergo changes in their kidney size and function when they migrate from saltwater to freshwater environments. 

Some birds like the barn swallow shrink the size of their digestive organs to reduce weight and enhance the size of their flight muscles before migration.

Also, since migratory animals are aware of photoperiod, they use it as a cue for migration. They exhibit migratory restlessness, a behavior characterized by increased activity and agitation in an animal when it’s time to migrate. 

In migratory birds, this behavior often includes repeated movements, such as flapping wings, hopping, or flying short distances. It typically occurs before the actual migration begins, helping prepare the animal for its long-distance travel.

“Birds clearly use photoperiod as a signal for ‘migratory restlessness’ at appropriate times of the year. Many animals (including salmon), particularly birds or even insects, can use magnetic field information to help migration,” said Kyriacou.

These adaptations have enabled migratory animals to prepare mentally and physically for migration in advance, allowing their circadian rhythms to smoothly adjust to time-zone-related changes without experiencing jet lags. However, it is important to note that there have been some cases where animals held in captivity showed signs of jet lag when moved to a new habitat. 

Humans on the other side, are not made for long-distance travel. Unlike migratory animals, we aren’t supported by evolution and have no special physiological adaptations for migration, and therefore, we’re destined to get jet lagged.