What does melatonin do? The answer may shock you!
Have you every wondered what does melatonin do? If you're like most people, you probably think it's a sleep hormone. But I'm going to let you in on a secret: It's not.
In humans, higher melatonin levels correlate with our sleep period. This means that levels rise as we go to bed. But it's a mistake to assume that melatonin is inducing sleep just because they share a correlation. Gratuitous correlation doesn't equal causation statement!
In mice and other nocturnal animals, melatonin follows the same environmental cycle: High at night and low during the day. But, in these animals, melatonin is high when they're alert and low when they sleep.
Thought it's clear we're not nocturnal, this points to melatonin as being a hormonal marker of environmental light, or darkness, if you will. It just so happens that we sleep when it's dark…when melatonin is high.
But this creates a ton of confusion. Many mistakenly believe that blocking blue light at night is essential to sleep and a strong circadian rhythm. They think this because blue light at night inhibits melatonin secretion.
Many researchers in the area of circadian research don't believe that melatonin is all that important of a sleep signal. At least on its own. The role of melatonin on circadian rhythms may act more to stabilize the rhythm in the face of abrupt changes.
On the other hand, the casual internet reader believes that melatonin is the most important signal for sleep. As a result, they're under the mistaken assumption that any inhibition of melatonin is the most important thing to address. This is why blue blocking glasses are all the rage.
But it's clear by the data that total light intensity is more important than blocking the blue wavelength of light at night.
A quick rundown of what melatonin does: Circadian rhythm edition
Let's begin this chat by stating something you may already know: melatonin is produced in many cells in your body. In fact, the largest source of melatonin in your body comes from your gut. Interestingly, this is regulated by feeding and not light exposure.
But don't worry, this melatonin mostly acts locally in the gut. Thus, it doesn't impact the way light regulates our circadian rhythm, described in the image below:
https://www.researchgate.net/publication/333354634_Melatonin_and_Melanopsin_in_the_Eye_Friends_or_Foes
As light enters the eye, it stimulates non-photo-forming cells known as intrinsically photosensitive retinal ganglion cells(ipRGCs). The light stimulates a photopigment called melanopsin found in the ipRGCs, which sends an action potential to the master circadian clock found in the suprachiasmatic nucleus(SCN) in the hypothalamus
This causes the SCN to inhibit melatonin secretion from the pineal gland. Though many wavelengths of light affect melanopsin, it's most sensitive to blue light. Therefore, blue light greatly inhibits melatonin secretion. When the Sun goes down and we're not exposed to blue light from artificial sources, the SCN instructs the pineal gland to make melatonin.
This is why people believe that blocking blue light is the most important light-related factor for circadian rhythms and sleep. Missing from the conversation is that rods and cones also project into the SCN, so it's unlikely that blue light is in and of itself the most important signal.
Additionally, the ipRGCs also receive input from rods and cones. In essence, they "act as "integrators of information" regarding the light environment across a wide range of wavelengths and light levels."
Thus, focusing only on blue light is a mistake, and probably doesn't do much.
Is melatonin the most important light-related factor for sleep and circadian rhythms?
But even if we assume that blue light is the most important signal, this is based on the notion that it has the greatest effect on inhibiting melatonin.
The only reason people think this is because it's easy to measure melatonin as a surrogate marker of circadian rhythms. So what happens to someone if they don't make pineal melatonin? In this instance, the SCN still creates a rhythm, the pineal just doesn't create a melatonin signal.
Fortunately, we have human data to give us a clue. In a paper published in 2016, researchers followed 8 individuals who had their pineal gland removed due to a tumor.
To confirm the absence of the pineal gland, they measured nighttime salivary melatonin levels. In all patients, melatonin was below the threshold for dim light melatonin onset, and in 5, melatonin was not detected.
Researchers assessed patients 11 days prior to removal of the pineal gland and approximately 8 months after surgery. They assessed activity patterns and underwent polysomnography to assess sleep.
What does melatonin do for sleep?
So what did the results of this study show? There was no significant difference in polysomnography scores measured before and after pinealectomy. In fact, after removal of the pineal gland, their sleep looked…well…normal.
See below for scores after pinealaectomy as well as the change from before to after
Sleep efficiency-95% (+1)
% REM Sleep-23% (-2)
%N1-3% (-1)
%N2-44% (No change)
%N3-29% (+1)
Sleep onset latency-17 mins (-4)
REM Latency-94 mins (+6)
Wake after sleep onset-27 mins (-2)
Stage changes-76(-3)
Frequency of night awakenings-18(+1)
Not only were these results not significantly different, both before and after fall within the normal range for all variables. Though this may seem surprising, it jibes with the supplemental melatonin research.
In fact, meta-analyses show fairly underwhelming benefits of supplemental melatonin for sleep. The largest showed a decrease in sleep onset latency of 7 mins and an increase of only 8 mins for total sleep time in people with sleep disorders.
Interestingly, people thought their sleep was worse after removing their pineal gland, but this was also not significant.
What does melatonin do for circadian rhythms?
In this paper, there were no significant difference in circadian activity and sleep profiles before and after pinealectomy. Though they didn't measure other circadian hormones, other lines of data find no change in the absence of pineal melatonin.
In their conclusion, they discuss these studies, which show "…that pineal melatonin depletion has no effect on diurnal variation of growth hormone, cortisol, adrenocorticotropic hormone, and thyroid-stimulating hormone (Kocher et al., 2006; Macchi and Bruce, 2004; Murata et al., 1998; Zeitzer et al., 2000)."
This doesn't mean that blue light is unimportant for circadian rhythms. In fact, blue light is still entering the eye and the SCN is still functional and creating a rhythm.
What it does show is that assuming blue light is the most important light-related factor for sleep and circadian rhythms is incorrect. It's based on the erroneous notion that inhibition of melatonin by blue light is the most important signal.
As it turns out, melatonin may function well as a biomarker for circadian rhythms, but it's not the only signal. Furthermore, eliminating the signal from the pineal gland doesn't abolish circadian rhythms. In fact, it doesn't even really appear to affect it to a significant degree.
Of course, it's important to point out this study only has a small number of subjects. And follow up with people who had pinealectomy years prior did show larger, but still insignificant, changes in sleep.
But being exposed to a small amount of blue light at night from a computer screen is certainly not going to inhibit pineal melatonin secretion nearly as much as removing the pineal gland altogether.
So what does melatonin do? Well, based on this data, it doesn't seem to be a particularly effective sleep hormone. And while it seems to be a good biomarker of circadian rhythm, it doesn't seem to have a robust effect there either. We can safely say it's a hormonal output of light exposure, but only a small part of the story.
It's tempting to think that maybe the master clock in the SCN regulates melatonin in tissues other than the pineal gland, But this doesn't explain the elimination of DLMO in all, and detectable salivary melatonin in 5, of the patients.
Clearly circadian rhythms are much more complex than most people realize. Simply throwing on a pair of blue blockers at night really isn't going to do much for anybody. That said, there is likely minimal harm in doing so if you like.
But if your goal is to optimize your circadian rhythms, your best bet is to string together and layer the dozen or more other behaviors that can have a sizeable impact. Things like:
Finding your ideal sleep and wake times
Maintaining a consistent schedule
Maintaining consistent habits
Getting more light during the day
Lowering the lights at night
Maintaining a healthy metabolism
Practice good sleep hygiene
Limit alcohol
Maintain nutritional adequacy
If your goal is to build a strong circadian rhythm to promote health, happiness, and performance, that's where you should focus your attention. Not wearing a pair of blue blocking glasses.
Posted in: Circadian Rhythms
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