Jet lag occurs when rapid travel across time zones desynchronizes your circadian rhythm — the internal 24-hour clock governed by the suprachiasmatic nucleus (SCN) in your brain. This master clock regulates sleep-wake cycles, hormone release, body temperature, and digestion. When your SCN receives light signals that conflict with its current phase, the result is the fatigue, insomnia, and cognitive fog known as jet lag. Our calculator models this desynchronization using published circadian research to predict your severity and generate a recovery timeline.

The direction of travel fundamentally changes how your body adapts. Westbound flights require you to delay your body clock, which aligns with the natural tendency of human circadian rhythms to run slightly longer than 24 hours. Eastbound flights force the opposite — advancing your clock — which explains why eastbound jet lag is roughly twice as severe. The calculator factors in this asymmetry when estimating your recovery days and generating light exposure windows.

Light is the most powerful signal for resetting your circadian clock. Bright light in the morning advances your clock (helpful after eastbound travel), while evening light delays it (helpful after westbound travel). The calculator uses your route's timezone shift and direction to determine the optimal light exposure windows for your specific flight, so you know exactly when to seek or avoid sunlight at your destination.