Jet Lag Disorder | Why Does Crossing Time Zones Disrupt Your Body?

Jet Lag Disorder is a temporary sleep problem that occurs when your body's internal clock is out of sync with a new time zone. This internal clock, known as the circadian rhythm, is a roughly 24-hour cycle that regulates critical functions, including the sleep-wake cycle, hormone release, and body temperature. The master controller of this rhythm is a tiny region in the brain's hypothalamus called the Suprachiasmatic Nucleus (SCN). The SCN functions like a central pacemaker, keeping all of your body's various biological clocks synchronized. It receives direct input from the eyes, using light as its primary signal to align the body's internal time with the external environment. When you are in your home environment, this system works seamlessly, making you feel alert during the day and sleepy at night. This precise, light-dependent regulation ensures that physiological processes occur at the optimal times.

When you rapidly travel across multiple time zones, your internal circadian rhythm remains aligned with your original location. However, the external cues of the new environment, especially the light-dark cycle, are now different. This mismatch is called desynchronization. Your SCN continues to signal for wakefulness or sleep according to the old time, leading to a conflict with the new local time. For example, if you fly from Seoul to Paris, your brain might be signaling that it's the middle of the night and time to sleep, while the local time in Paris is the middle of the afternoon. The body requires time to reset, or "entrain," its internal clock to the new schedule, and it is during this adjustment period that the symptoms of jet lag manifest.

The desynchronization of the circadian rhythm affects nearly every system in the body, leading to a variety of symptoms. The most common include significant daytime fatigue, an inability to stay alert, and insomnia or disturbed sleep at night. Cognitively, you may experience difficulty concentrating and a general feeling of being unwell. Gastrointestinal issues, such as indigestion or constipation, are also frequent, as the digestive system's rhythm is disrupted. Mood changes, including irritability and a lack of motivation, can also occur because the hormones that regulate mood are also tied to the circadian cycle.

Light is the most powerful environmental cue for resetting the SCN and, consequently, your circadian rhythm. Strategic exposure to light can significantly speed up your adaptation to a new time zone. When traveling east, you need to "advance" your body clock. To do this, expose yourself to bright light, preferably sunlight, in the morning of your new location. When traveling west, you need to "delay" your clock. This is achieved by seeking bright light in the late afternoon and early evening at your destination. Conversely, avoiding light at the wrong times—such as wearing sunglasses to block morning light when traveling west—is just as crucial to prevent sending confusing signals to your brain.

Most individuals find it harder to adjust after flying east than after flying west. This is because the human body's intrinsic circadian period is naturally slightly longer than 24 hours (around 24.2 hours). As a result, our bodies have a natural tendency to delay sleep and wake times. Traveling west lengthens your day, which aligns with this natural tendency to "phase delay," or drift later. In contrast, traveling east shortens your day, requiring a "phase advance," where you must force your body to sleep and wake earlier than it is prepared to. This is biologically more challenging and requires more time for the SCN to adjust. The general rule of thumb is that the body can typically adjust about one time zone per day for westward travel, but only about one time zone every one-and-a-half days for eastward travel.