It is a temporary sleep problem that usually occurs when you travel across more than three time zones but can affect anyone who travels across multiple time zones. Jet lag can affect your mood, your ability to concentrate, and your physical and mental performance.

Jet Lag Syndrome, also known as desynchronosis, is a temporary sleep disorder that occurs when your body's internal clock (circadian rhythm) is out of sync with the local time of your destination after crossing multiple time zones. Your body's "master clock" in the brain uses sunlight to regulate when you feel awake or sleepy; when you fly across the world, your body still thinks it’s in London while you are actually in Tokyo. Symptoms include extreme daytime fatigue, insomnia, digestive issues, and "brain fog." Generally, it takes about one day of recovery for every time zone crossed. In 2026, bio-travelers use "light therapy" and specific apps (like Timeshifter) to gradually shift their sleep schedules before they even board the plane. It is medically different from simple travel exhaustion; while exhaustion goes away with a good night's sleep, true jet lag persists for several days as your body's hormonal cycles, like melatonin and cortisol production, slowly realign with the natural light-dark cycle of your new location.