Your body clock runs long, so flying east is an uphill battle ⏰

The human circadian system — the internal clock that governs sleep, alertness, hormone release and body temperature — has an intrinsic period that is close to, but not exactly, 24 hours. In carefully controlled experiments without time cues, most people settle into a day of about 24.2 hours, meaning the clock naturally drifts later rather than earlier. ^[1][2]

That slight daily lengthening is why westward travel is usually easier. Flying west extends the day, which matches the clock's tendency to run long, so the body can delay sleep and wake times with relative ease. Flying east compresses the day, forcing the clock to advance — to go to bed and wake up earlier than it wants. This is the harder direction for most travelers. ^[1][3]

Why the clock runs long

The master clock sits in the suprachiasmatic nucleus of the hypothalamus and is normally synchronized by light. In the absence of light cues, the clock free-runs at its own pace. Classic studies by researchers such as Jürgen Aschoff and later Charles Czeisler showed that the human free-running period averages slightly over 24 hours, though individual values vary. ^[2][4]

What this means for jet lag

Jet lag is the mismatch between the internal clock and the external day after rapid travel across time zones. Because the clock resists advancing, eastward flights typically produce more severe and longer-lasting symptoms than westward flights of the same number of time zones. The rule of thumb is that adaptation takes roughly one day per time zone, but eastward adaptation can take longer. ^[3][5]

• Westward travel: the day is longer, matching the clock's natural drift, so delaying sleep feels easier. ^[1][3]
• Eastward travel: the day is shorter, requiring an advance that the clock resists, so falling asleep and waking earlier is harder. ^[1][3]
• Light exposure is the strongest resetting signal; timed light and avoidance of light can speed adaptation. ^[4][5]
• Melatonin taken at the right local time can help shift the clock, but timing matters and should be discussed with a clinician. ^[5]

Practical implications

Travelers heading east often benefit from starting the shift a few days before departure by going to bed and waking earlier, and by seeking morning light at the destination. Those heading west can more easily stay up later and use evening light to delay the clock. The asymmetry is not a matter of willpower; it reflects the underlying biology of a clock that prefers a slightly longer day. ^[3][4][5]