How the body tells time, and why night shifts do harm

This is the long version of §2.39 (the longer you work night shifts, the higher your cardiovascular risk). That entry only does the accounting; this article explains the reasoning behind it.

The question to answer: can the body really tell what time it is? And if it can, why is night work not something you "get used to after enough late nights"?

1. There really is a timekeeping organ in the body

In the human brain's hypothalamus there is a small cluster of neurons called the suprachiasmatic nucleus, about 20,000 (2万) cells. It is the body's master clock.

It keeps time through a set of "clock genes" that suppress each other. Two proteins, CLOCK and BMAL1, switch on the transcription of two other genes, PER and CRY. Once the PER and CRY proteins have built up, they turn around and switch off the pair of proteins that turned them on. When those proteins have broken down, the cycle starts over. One full loop takes about 24 hours.

So this clock does not work out the time by watching the outside world. Put a person in a room with no light, no clocks and no social cues at all, and the rhythm keeps running. It just isn't exactly 24 hours, and it drifts slowly day by day. The 2017 Nobel Prize in Physiology or Medicine went to the three researchers who discovered this molecular mechanism.

2. The clock is set by light, but not through the "seeing" pathway

Besides the cells that form images, the retina has a special class of light-sensing cells. They contain a pigment called melanopsin. They play no part in seeing things clearly; they send the strength of the light straight to the suprachiasmatic nucleus.

So whether light can calibrate the body clock has no direct connection to whether the eyes can see. Some blind people who see nothing at all still have body clocks that light can calibrate.

This is also why §3.2 (get up at the same time every day, weekends included) should be paired with light in the morning.

3. The key point: there is more than one clock

The liver, gut, heart and fat tissue each have their own set of peripheral clocks.

The master clock is set by light. Peripheral clocks are set largely by when you eat.

On a normal schedule, the two are in sync. You see light in the day and eat in the day, and the master clock and the peripheral clocks point to the same time. So ordinarily you never notice that they are two separate systems.

4. The problem with night shifts is not "staying up" but that the two clocks get pulled apart

People who work nights still see light in the daytime and still live a daytime social life. So the master clock mostly stays locked to the outside day-night cycle.

But you eat, work and fire up your metabolism in the biological middle of the night. The peripheral clocks get pulled to the other side.

This state is called circadian misalignment. The problem is not that you slept a few hours less, but that the clocks across the body no longer match each other.

5. And you can't adapt

Some people think that if you work permanent nights for long enough, you'd surely adapt.

One review pooled six studies that used the melatonin secretion rhythm to test people on permanent night work. The melatonin rhythm is the most reliable indicator of the state of the internal body clock.

The result: fewer than 3% achieved "complete" adaptation. Fewer than a quarter reached "considerable" adaptation.

It made no difference how many years they had worked. The reason is in the previous paragraph: you still see light in the day and still live a daytime social life, and these cues keep pulling the master clock back.

Source: Folkard S (2008). Do permanent night workers show circadian adjustment? A review based on the endogenous melatonin rhythm. Chronobiology International. https://doi.org/10.1080/07420520802106835

6. How do we know misalignment itself does the harm, rather than just less sleep

This is the most worthwhile question in the whole matter. People who work nights sleep less, eat irregularly, are under more stress and smoke more, and any of these could be the real cause.

Fortunately, this can be isolated in a laboratory.

The 2009 study by Scheer et al.: 10 adults lived in a laboratory for 10 days. The researchers artificially set the "day" to 28 hours, forcing their eating and sleeping to cycle through every point of the body clock. Four meals a day, with fixed calories.

When behavior and the body clock were about 12 hours apart, the results were:

• Leptin fell 17% (leptin is the hormone that makes you feel full)
• Blood sugar rose 6%, even though insulin had risen 22%
• The daily rhythm of cortisol was completely inverted
• Mean arterial pressure rose 3%
• Sleep efficiency fell 20%

The most important point: of the 8 people with enough data, 3 had after-meal blood sugar that entered the prediabetes range, in only 10 days.

Because calories were fixed and the experiment was controlled in a lab, these changes can't be put down to "less sleep" or "eating more." Misalignment alone is enough to cause these changes.

Source: Scheer FA, Hilton MF, Mantzoros CS, Shea SA (2009). Adverse metabolic and cardiovascular consequences of circadian misalignment. PNAS, 106(11), 4453-4458. https://doi.org/10.1073/pnas.0808180106

7. The first thing you can change: move your eating to the daytime

Since peripheral clocks are set by when you eat, there is one direction you can try: work the night shift but don't eat at night.

A randomized controlled trial did this (registration number NCT02291952). 20 healthy people simulated night work and were randomly split into two groups. One group ate at night and in the day, as night workers commonly do. The other group ate only in the daytime.

The main-outcome report said: in the group that ate at night, the central rhythm and the peripheral glucose rhythm were pulled apart and glucose tolerance got worse; the group that ate only in the daytime showed neither of these.

The cardiovascular paper (a secondary analysis of the same trial) said: in the night-eating group, the cardiac vagal index pNN50 fell 25.7% and RMSSD fell 14.3%, LF/HF, which reflects the balance of sympathetic and vagal activity, rose 5.5%, and PAI-1, a clot-promoting factor, rose 23.9%. In the group that ate only in the daytime, these changes did not appear.

But this was only 20 people, and it simulated night work rather than using real night workers.

A systematic review that looked specifically for this kind of randomized trial searched six databases and found only 4 trials. The results were not tidy either: some trials saw improvement in fasting insulin and insulin resistance and some did not; no change in high-density lipoprotein was seen in any of the four; for after-meal blood sugar, a 10.75-hour overnight fast did do better than eating at night with a late-night snack. The review authors themselves said there were too few trials to generalize.

So all that can be said for now is this: concentrating your eating in the daytime during night shifts is supported by randomized trials, but the evidence is still thin. It costs nothing and isn't hard to do, so try it if you like. There is a second thing to try, in the next section: using light to move the whole body clock over.

Sources: Chellappa SL et al. (2021). Daytime eating prevents internal circadian misalignment and glucose intolerance in night work. Science Advances. https://doi.org/10.1126/sciadv.abg9910; Chellappa SL et al. (2025). Daytime eating during simulated night work mitigates changes in cardiovascular risk factors: secondary analyses of a randomized controlled trial. Nature Communications. https://doi.org/10.1038/s41467-025-57846-y; Wulandari F et al. (2026). Cardiometabolic status among shift workers under meal time regulation: A systematic review of randomized controlled trials. Nutrition and Health. https://doi.org/10.1177/02601060261464874

8. Can you move the clock over?

All of the above was about "misalignment is harmful." So conversely, can you move the whole clock over to the night-shift side?

In theory yes, and it has been done. But the light it needs is an order of magnitude brighter than most people think.

The 1990 study by Czeisler et al.: 8 young men worked a week of night shifts under two conditions.

In the control condition, the night lighting was ordinary indoor light, about 150 lux. Lux is the unit of illuminance; a room with ordinary fluorescent lights is roughly one or two hundred. The result was that after six nights in a row, the lowest point of body temperature still fell around 3:30 a.m. The temperature minimum is the standard marker of where the body clock points. This shows that the body clock had not budged at all.

In the treatment condition, they were given bright light of 7,000 to 12,000 lux at night and slept in near-total darkness during the day. After four days, the temperature minimum had moved to nearly 3 p.m. That is, the whole thing had flipped.

Source: Czeisler CA, Johnson MP, Duffy JF, Brown EN, Ronda JM, Kronauer RE (1990). Exposure to bright light and darkness to treat physiologic maladaptation to night work. New England Journal of Medicine, 322(18), 1253-1259. https://doi.org/10.1056/NEJM199005033221801

The researchers recommend doing all three together

First, the light at night has to be bright enough. Working under ordinary fluorescent lights is the same as the control group above, in which "nothing happened." To work, you need a dedicated light therapy box.

Second, block the light strictly when you sleep during the day. Blackout curtains plus an eye mask. The experiment above did "bright light at night" and "near-total darkness by day" together; it was not one or the other.

Third, wear sunglasses on the way home. This is the one most easily left out. In a 1994 experiment with 50 people, bright light and sunglasses each worked on their own, and both together worked best; without sunglasses, the body clock shifted only in the earlier direction and could not shift to the position that daytime sleep needs. See the sources section of §2.39 (if you can switch off night work, do it early) for the source.

To see why the third matters so much, look at how light acts: the same light, falling at different times, pushes the body clock in opposite directions. Light before the temperature minimum pushes the clock later; light after it pulls the clock earlier.

When you walk home at the end of a night shift, it is just after the temperature minimum. What that morning light does is the opposite in direction of what you did sitting in front of the light box all night.

Possible in a lab, not possible in daily life

The people in the lab did not go outside for a week. In real life, on your days off you live a daytime life, picking up children, spending time with family, running errands. Once is enough to pull the clock back. That is where the number in section 5 comes from: fewer than 3% of long-term permanent night workers adapt completely.

So in the real world you usually don't aim to flip completely, but only to move part of the way, so that the clock rests at a middle position where "you're alert enough at work, and your days off aren't wrecked."

How much you can get in practice

A meta-analysis pooling 11 studies: light therapy gave shift workers about 32.5 more minutes of total sleep and raised sleep efficiency by about 2.91. The effective dose was 900 to 6,000 lux for at least 1 hour each night.

Another lab study used intermittent bright light. In the treatment group, melatonin secretion overlapped the scheduled sleep period by 4.90 hours, against only 2.62 hours in the control group (P=0.002). The decline in cognitive performance that the control group showed on the last night shift was held down in the treatment group. The authors state in their conclusion that this approach still needs to be verified in real night workers.

Sources: Zhao C, Li N, Miao W, He Y, Lin Y (2025). A systematic review and meta-analysis on light therapy for sleep disorders in shift workers. Scientific Reports. https://doi.org/10.1038/s41598-024-83789-3; Lammers-van der Holst HM et al. (2021). Efficacy of intermittent exposure to bright light for treating maladaptation to night work on a counterclockwise shift work rotation. Scandinavian Journal of Work, Environment & Health. https://doi.org/10.5271/sjweh.3953

One thing nobody has measured

"If you move the clock correctly, does cardiovascular risk fall with it?" No trial has measured this.

What is known is that misalignment is harmful, as shown by the experiment in section 6. But that does not mean the reverse, that "moving it correctly is safe." And whether moving the clock back again on days off, going back and forth like this, is good or bad has likewise not been measured.

The endpoints of the two measures above, one reaching metabolic and cardiovascular markers and the other reaching sleep, never got as far as disease and death.

9. What this article does not cover

How much night work raises cardiovascular risk, and how it scales with years, is in §2.39 (the longer you work night shifts, the higher your cardiovascular risk).

How to make up sleep after staying up late is in §2.38 (catch up on sleep the night after a late night).

How long to sleep at night and how regular your schedule is are in §2.13 (sleep about 7 hours a night on a regular schedule).

Morning light and a fixed wake-up time are in §3.2 (get up at the same time every day, weekends included).