Body clock
Your body clock, and how light and melatonin move it earlier or later.
A tiny clock deep in the brain decides when you feel sleepy and when you wake up. Light sets it every day, and darkness lets your body make melatonin at night. See how it works, why some people's clocks run early or late, and how well-timed light and melatonin can move it.
1 Light sets the clock
Melatonin: Off
Daylight reaches special light-sensing cells at the back of the eye. They send a steady daytime signal along a nerve to the clock, a tiny cluster of cells just above where the two optic nerves cross, called the suprachiasmatic nucleus, or SCN. While that signal is on, the pineal gland stays quiet and melatonin stays low.
2 Early birds, night owls and everyone between
- This sleeper's melatonin
- Sleep
- Body's low point
- 7 am alarm
A typical clock
- Melatonin starts
- about 9 pm
- Sleep comes easily
- 11 pm to 7 am
- Body's low point
- about 4:30 am
Melatonin starts around 9 pm, sleep comes easily around 11 pm, and the body reaches its low point, its coolest temperature of the day, around 4:30 am. A 7 am alarm rings after the body's night is mostly over.
3 Move the clock
Light and melatonin are the two signals the clock listens to most. The same signal can move the clock earlier or later, depending on when it arrives by the body's own clock, not the clock on the wall. Pick a sleeper, set the times, and step through a week.
- Melatonin now
- Where it started
- Moves it earlier
- Moves it later
- Barely moves it
The two strips under the curve show what bright light (sun) and melatonin (pill) would do at each hour for this clock. They slide with it.
Pulls the clock earlier, about 15 min a day.
Pulls the clock earlier, about 10 min a day.
Before: no light box, no melatonin.
Sleep comes easily around 3 am.
Sleep diary: when sleep comes easily, day by day
Why timing matters more than brightness
The turning point for light is the body's low point, its coolest temperature of the night, which usually falls about two to three hours before you would wake up on your own. Light in the hours before it pushes the clock later. Light in the hours after it pulls the clock earlier. That is why a night owl who forces a 6 am walk in bright light can end up later, not earlier: for their clock, 6 am is still the middle of the night.
Melatonin works on a different schedule. It moves the clock most when taken in the afternoon or early evening, a few hours before your own melatonin would start, and it barely moves the clock at bedtime, when it mostly makes you feel sleepy. Used together, morning light and earlier melatonin add up.
Using this in real life
A real plan moves in small steps, keeps the wake time fixed seven days a week, and dims the lights in the evening, because every lie-in and late screen pulls the other way. The articles on delayed sleep phase and advanced sleep phase walk through the timing used in studies, and the guide to light therapy covers light boxes. Talk with your doctor before starting melatonin, especially if you take other medicines or are pregnant.
A teaching model, not a measurement of anyone's clock. Each sleeper's clock is set by one time, when their melatonin starts, with sleep 2 hours later, the body's low point 7.5 hours later and waking 10 hours later. The light curve follows the shape measured by Khalsa and colleagues (2003) and St Hilaire and colleagues (2012): light centred before the low point delays the clock, light after it advances it. The melatonin curve follows the low dose (0.5 mg) curve of Burgess and colleagues (2010), with its biggest advance 2 to 4 hours before melatonin onset, little effect across the first half of the night, and a smaller delay after waking. Sizes are set on the cautious side: at their best times, light here moves the clock up to about 15 minutes a day and melatonin up to about 10, less than the 1.7 hours in three days that morning light with an earlier schedule produced in healthy volunteers (Revell and colleagues, 2006). People with a delayed clock often respond more slowly. Everything else about the day is held constant, so the model leaves out evening screens, lie-ins and the clock's own tendency to run a little late. The real path from the clock to the pineal gland runs down to the upper spinal cord and back up through the neck; the dashed line in the drawing stands for that relay. The melatonin curve is its usual shape in dim light, drawn without units because real levels differ many fold from person to person. Nothing here is a treatment plan for any one person.
Sources
- Auger RR, Burgess HJ, Emens JS, et al. Clinical practice guideline for the treatment of intrinsic circadian rhythm sleep-wake disorders: an update for 2015. J Clin Sleep Med. 2015;11(10):1199-1236. doi:10.5664/jcsm.5100
- Khalsa SBS, Jewett ME, Cajochen C, Czeisler CA. A phase response curve to single bright light pulses in human subjects. J Physiol. 2003;549(Pt 3):945-952. doi:10.1113/jphysiol.2003.040477
- St Hilaire MA, Gooley JJ, Khalsa SBS, et al. Human phase response curve to a 1 h pulse of bright white light. J Physiol. 2012;590(13):3035-3045. doi:10.1113/jphysiol.2012.227892
- Zeitzer JM, Dijk DJ, Kronauer R, Brown E, Czeisler C. Sensitivity of the human circadian pacemaker to nocturnal light: melatonin phase resetting and suppression. J Physiol. 2000;526(Pt 3):695-702. doi:10.1111/j.1469-7793.2000.00695.x
- Burgess HJ, Revell VL, Eastman CI. A three pulse phase response curve to three milligrams of melatonin in humans. J Physiol. 2008;586(2):639-647. doi:10.1113/jphysiol.2007.143180
- Burgess HJ, Revell VL, Molina TA, Eastman CI. Human phase response curves to three days of daily melatonin: 0.5 mg versus 3.0 mg. J Clin Endocrinol Metab. 2010;95(7):3325-3331. doi:10.1210/jc.2009-2590
- Revell VL, Burgess HJ, Gazda CJ, et al. Advancing human circadian rhythms with afternoon melatonin and morning intermittent bright light. J Clin Endocrinol Metab. 2006;91(1):54-59. doi:10.1210/jc.2005-1009
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