Sleep Tools
Jet Lag Calculator
Enter your origin and destination time zones, your direction of travel, and when you land — and get a personalised recovery plan with exact sleep windows, light exposure timing, and how many days until your body clock fully adjusts.
10 time zones
Eastward travel — clock must advance
15
days to full recovery
Zones Crossed
10h
Direction
East →
Recovery
~15 days
What Is Jet Lag?
Jet lag — formally called desynchronosis — is a temporary sleep disorder that occurs when your body's internal clock is out of sync with the local time at your destination. Your circadian rhythm, which regulates sleep, hunger, hormone release, and body temperature, operates on a roughly 24-hour cycle anchored to the light and dark patterns of your home location.
When you cross multiple time zones rapidly, your circadian rhythm doesn't move with you. It continues running on home time. The result is a mismatch — sometimes called internal desynchrony — between what your body expects and what the environment is doing. Your brain is telling you to sleep at 2 PM local time, or to be wide awake at 3 AM.
The severity depends on how many zones you cross, which direction you travel, your individual chronotype, your age, and what you do on the plane and in the first 48 hours after landing. Crucially, jet lag is not simply tiredness from a long flight — it is a genuine circadian rhythm disruption with measurable physiological effects that persist for days.
Symptoms of Jet Lag
Jet lag affects nearly every physiological system — not just sleep. The symptoms below are documented in clinical literature and occur because multiple body systems each run on their own circadian clock, and they all need time to resynchronise independently.
Daytime Sleepiness
Intense fatigue during destination daytime hours while your body thinks it should be asleep.
Nighttime Insomnia
Inability to fall or stay asleep when local time says you should be sleeping.
Cognitive Fog
Impaired concentration, slow reaction time, and difficulty with complex decisions.
Mood Disruption
Irritability, low mood, and emotional dysregulation — driven by cortisol and melatonin being out of phase.
GI Disruption
Nausea, appetite loss, constipation, or diarrhoea as your gut microbiome also follows a circadian rhythm.
General Malaise
Headaches, muscle weakness, and a general sense of feeling unwell — similar to mild illness.
Why East Is Harder Than West
The direction of travel has a profound effect on jet lag severity. Flying eastward requires your circadian clock to advance — to shift earlier. Flying westward requires it to delay — to shift later. The human circadian clock has a natural period slightly longer than 24 hours, which makes delaying easier than advancing.
Think of it this way: staying up late feels manageable, but waking up 3 hours earlier than usual feels genuinely difficult. That asymmetry is biological. Research consistently shows that eastward jet lag is roughly 50% more severe per time zone and takes proportionally longer to resolve.
Flying East — Clock Advance
Harder • ~1.5 days per time zone
- →Must fall asleep earlier than your body wants
- →Waking up feels impossible at local time
- →Morning grogginess is more severe
- →Recovery takes longer per zone crossed
- →Strategy: morning light at destination
Flying West — Clock Delay
Easier • ~1 day per time zone
- →Must stay awake later than usual
- →Waking up naturally too early at destination
- →Daytime fatigue is the main symptom
- →Recovers faster — body clock delays more easily
- →Strategy: evening light at destination
How to Recover From Jet Lag Faster
Jet lag is not inevitable — it is a circadian timing problem, and circadian timing can be influenced. These six strategies are the most evidence-backed interventions available, ordered by impact.
Time Your Light Exposure
Light is the most powerful circadian signal. Get outdoor light at the right time for your direction of travel — morning light for eastward trips, evening light for westward. Avoid bright light at the wrong phase; use an eye mask or blackout curtains if needed.
Use Melatonin at the Right Time
A low dose (0.5–3mg) of melatonin taken at destination bedtime for the first 3–4 nights significantly accelerates adjustment. Timing matters more than dose — melatonin taken at the wrong time of day can worsen jet lag.
Stay Awake Until Local Bedtime
Anchor to your destination's schedule immediately on arrival. Resist the urge to nap for more than 20 minutes during the day. A short power nap is fine to reduce acute sleepiness, but long naps will delay your circadian adjustment.
Adjust Sleep Before You Fly
Starting 2–3 days before a major eastward flight, go to bed and wake 30–60 minutes earlier each day. For westward, shift later. This pre-adjustment reduces the total adjustment load on arrival.
Avoid Alcohol on the Flight
Alcohol may help you feel drowsy but it severely fragments sleep quality — reducing REM and deep sleep. The dehydration effect at altitude compounds this. Water and electrolytes are far better choices on long-haul flights.
Eat on Destination Time
Your digestive system follows its own circadian rhythm. Eating at destination mealtimes — not your home timezone's mealtimes — sends an additional timing signal to your peripheral clocks and speeds up overall adjustment.
The Science of Jet Lag Recovery
Your circadian rhythm is set primarily by the suprachiasmatic nucleus (SCN) — a small cluster of roughly 20,000 neurons in the hypothalamus that acts as your master clock. The SCN synchronises to environmental light via retinal photoreceptors called intrinsically photosensitive retinal ganglion cells (ipRGCs), which are most sensitive to blue-wavelength light.
When you travel across time zones, the SCN is still anchored to your origin time. It begins adjusting through light exposure, but the adjustment rate is limited — roughly 1–1.5 hours per day. This is why recovery is measured in days, not hours. A 9-hour eastward time zone difference (New York to Tokyo) requires the clock to advance by 9 hours, which takes approximately 6–9 days at maximum adjustment speed.
Peripheral clocks — in the liver, gut, muscles, and other organs — follow the master SCN clock but can be influenced independently by meal timing and activity. This is why eating on destination time accelerates full recovery: it helps peripheral clocks reset while the SCN is still adjusting. Melatonin, secreted by the pineal gland, is the SCN's primary timing signal to the rest of the body — which is why exogenous melatonin at the right time is one of the most effective jet lag interventions available without a prescription.
Frequently Asked Questions
How long does jet lag last?↓
Jet lag typically lasts one to one and a half days per time zone crossed. Travelling eastward tends to produce more severe and longer-lasting jet lag than travelling westward. A 6-hour eastward time zone difference can result in 6–9 days of disruption without intervention.
Is jet lag worse flying east or west?↓
Flying east is significantly harder for most people. When flying east, you must advance your circadian clock — essentially waking and sleeping earlier than your body wants. When flying west, you delay your clock, which is more natural. Research suggests eastward jet lag is roughly 50% more severe.
What is the fastest way to recover from jet lag?↓
The most effective interventions are timed light exposure, strategic sleep scheduling, and melatonin at the right time. Getting outdoor light at your destination in the morning (for eastward travel) or evening (for westward travel) is the strongest circadian signal available.
Does melatonin help with jet lag?↓
Yes, when timed correctly. A meta-analysis in the Cochrane Database found melatonin taken at the destination bedtime significantly reduces jet lag severity. For eastward travel, take 0.5–3mg at local bedtime for the first 3–4 nights. Timing matters more than dose.
Should you sleep on a red-eye flight?↓
For most travellers, sleeping on the plane is beneficial — especially on long eastward flights. Use an eye mask and earplugs, avoid alcohol, and try to time your sleep to match your destination's night hours. Avoid long naps if arriving in daylight heading westward.
How many time zones cause jet lag?↓
Most people begin to notice jet lag with 2 or more time zone changes. Below 2 zones, the body adapts without significant disruption. Above 5 zones, jet lag becomes severe and a structured recovery plan is strongly recommended.
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Content reviewed and updated March 2026. Based on published circadian rhythm and sleep medicine research.