Larks, owls and social jet lag
Some body clocks run early and some run late, and much of that is inherited. Learn your type, measure how far your days off drift from your workdays, and shrink the gap.
Chronotype
Your body clock's natural timing: when, left to yourself, you'd fall asleep, wake up and feel sharpest. Early types are often called larks, late types owls. Most people sit somewhere in the middle, leaning a little one way.
Two weeks into a holiday with no alarm, Maryam is asleep by 10:30pm and up at 6:30am. Her brother drifts to 2am and 10am. Same family, same holiday, different chronotypes.
WHERE IT COMES FROM
Genes, age and light
Chronotype is partly genetic; it runs in families. It also moves with age: it shifts later through the teenage years, peaks around age 20, then creeps earlier for the rest of your life. And your light habits push it around, as you saw in lesson 4. You can usually nudge it by an hour or so. You can't turn an owl into a lark by willpower.
A 16-year-old who can't fall asleep before midnight and can't think at 7am is mostly showing normal teenage biology, not a bad attitude.
Two ends of the range
Lark
Wakes easily, often before the alarm. Sharpest in the morning. Fades in the evening and struggles at late dinners and night shifts. School and office hours fit well.
Owl
Slow, foggy mornings. Sharpest in the late afternoon and evening. Not sleepy at a 'sensible' bedtime, so early starts mean short nights all week and long lie-ins on days off.
Check yourself
Kaveh is a clear owl. His new job starts at 7:30am. Which plan is realistic?
- Get into bed at 9pm every night and lie there until sleep comes, however many hours it takes
- Use morning light, dim evenings and a steady wake time to shift about an hour earlier
- Accept that the real problem is discipline, and push himself harder every morning
- Sleep 5 hours on workdays and make all of it up with long lie-ins on days off
Show the answer
Use morning light, dim evenings and a steady wake time to shift about an hour earlier
Yes. Light and a consistent wake time can move an owl's clock by an hour or so, and keeping it on days off stops it sliding back. That's a real gain without fighting his biology head-on.
THE HIDDEN COST
Social jet lag
When your work schedule and your body clock disagree, you live in two time zones: one on workdays, one on days off. The gap between them is called social jet lag. You measure it with the midpoint of your sleep, the clock time halfway between falling asleep and waking. The further the midpoint moves on days off, the bigger the jet lag.
Your body can't tell the difference between flying three time zones west for the weekend and simply sleeping three hours later. Either way it has to shift, then shift back.
Workdays: asleep 00:00, up 06:30
midpoint = 03:15
Days off: asleep 02:30, up 10:30
midpoint = 06:30
social jet lag = 06:30 - 03:153 hours 15 minutesThat's like crossing three time zones and back, every single week. Now work out your own: what were your two midpoints last week?
Check yourself
Nasrin sleeps 11pm to 7am on workdays and 3am to 11am on days off. Because she gets 8 hours every single night, the late days off cost her nothing.
Show the answer
False
False. Her hours are fine, but her timing swings by four hours, which is four time zones of jet lag every week. Expect trouble falling asleep before the first workday and a rough first morning. Timing matters as well as amount.
Why the first workday hurts
- Night before the day off
No alarm tomorrow, so you stay up until 2am under bright lights. That evening light pushes the clock later.
- Morning of the day off
You sleep until 11. You've missed the morning light that would have pulled the clock back, so it drifts later still.
- Last night before work
You go to bed at 11pm to be sensible. But you've only been awake 12 hours, so pressure is low, and your gate has moved to 1am. You lie there.
- First workday, 6:30am
The alarm goes off at what your clock thinks is the middle of the night. It takes until midweek to shift back, and then the next day off arrives.
Check yourself
- Hamid sleeps 11:30pm to 6:30am on workdays and 12:30am to 8am on days off. His first workday feels normal.
- Sina sleeps 11:30pm to 6:30am on workdays and 3am to 11am on days off. He can't sleep the night before work and feels wrecked the first morning.
Both sleep longer on days off. What makes the difference?
- Sina sleeps too many hours on his days off
- Hamid simply needs less sleep than Sina
- Sina's sleep timing shifts by hours; Hamid's moves by about one
- Hamid goes to bed before midnight on workdays
Show the answer
Sina's sleep timing shifts by hours; Hamid's moves by about one
Right. A one-hour drift is small enough for the clock to absorb. A shift of three or four hours is real jet lag, and the clock needs days to come back.
Check yourself
Roya was at a wedding until 2am on her night off. Her normal wake time is 7am and she works the day after tomorrow. What's the best plan for tomorrow?
- Get up by 8:30, get morning light, nap briefly after lunch, bed at the usual time
- Stay up all of the following night to 'reset' her clock in one go
- Go to bed at 6pm tomorrow to make up for the lost hours
- Sleep until 1pm to get the full 8 hours back, since the amount is what matters most
Show the answer
Get up by 8:30, get morning light, nap briefly after lunch, bed at the usual time
Yes. She protects the timing of her clock with a near-normal wake time and morning light, takes the edge off with a short nap, and lets the extra sleep pressure give her a solid night at the normal hour.
If you're an owl in a lark's world
- Use the tools that move a clock earlier: morning outdoor light, dim evenings, a steady wake time.
- Where you have a choice, fit the day to your type: later start times, hard thinking in your sharp hours.
- Stop reading your type as a moral failing. Owls who sleep enough do as well as larks.
- If your natural sleep time is so late that you can't hold a job or study, and nothing shifts it, a sleep doctor can help.
Check yourself
Does it widen or shrink social jet lag?
- Sleeping until noon on days off
- A walk outside at 8am on your day off
- A film marathon until 3am because tomorrow is free
- A 20-minute nap after lunch instead of a lie-in
- Waking within an hour of your workday time
- Shutting the blackout curtains 'to recover' all morning
Show the answer
Widens the gap: Sleeping until noon on days off, A film marathon until 3am because tomorrow is free, Shutting the blackout curtains 'to recover' all morning
Shrinks the gap: A walk outside at 8am on your day off, A 20-minute nap after lunch instead of a lie-in, Waking within an hour of your workday time
Lesson recap
- Chronotype is your clock's natural timing. It's partly genetic, shifts with age, and isn't a character trait.
- You can nudge it about an hour with light and a steady wake time, not flip it.
- Social jet lag is the gap between your sleep midpoint on workdays and on days off.
- Keep your wake time within about an hour every day; repay lost sleep with earlier nights and short naps.