False YouTube post

 I watched a YouTube video that asserted the function of sleep is to clear plaque out of the brain and that this can be defeated if one doesn't get at least ten minutes of sunlight first thing each morning.  It warned against other forms of light and asserted not doing this is a major cause of dementia.  I am skeptical.

Your skepticism is justified. The video appears to have taken several genuine pieces of neuroscience and welded them into a much stronger claim than the evidence supports.

There really is a connection between sleep and clearance of substances associated with Alzheimer's disease. The so-called glymphatic system is involved in moving substances out of brain tissue, and a number of studies suggest that sleep—particularly slow-wave/NREM sleep—facilitates clearance of amyloid-β and tau. A recent human study actually found evidence consistent with increased overnight clearance of these proteins during sleep.

But even that isn't as settled as YouTube presentations often make it sound. The glymphatic hypothesis is still being debated, and reviews of the human evidence have found inconsistencies. One recent review, for example, concluded that the evidence doesn't yet establish the simple proposition that slow-wave sleep is essentially a nightly "brain washing cycle."

The sunlight claim is where the video seems to go off the rails.

Morning light absolutely does something important: it helps synchronize the circadian clock. Bright light in the morning can influence the timing of melatonin secretion, sleepiness, alertness, and the sleep-wake cycle. That's a perfectly respectable reason to get outside in the morning.

But I can find no good evidence for the specific proposition that you need ten minutes of natural sunlight immediately after waking in order to activate brain plaque clearance, nor that failing to do so is a major cause of dementia.

Indeed, the Alzheimer's Society currently says that more evidence is needed before light therapy can even be recommended for treating dementia; bright light appears useful primarily for regulating circadian rhythms and associated sleep disturbances.

And the warning against "other forms of light" is particularly suspicious. Your circadian system doesn't have a little sensor that says, Ah, photons from the Sun—excellent; photons from a window or lamp—DENIED. The important variables are intensity, spectrum, timing, duration, and whether the light reaches the retina. Natural outdoor light is extraordinarily bright, so it is a convenient and effective way of getting a strong morning signal, but that doesn't make sunlight uniquely capable of doing it.

There's an important distinction here:

Good claim:
Poor sleep, especially chronic sleep disruption, is associated with increased Alzheimer's risk, and sleep appears to participate in maintaining brain protein homeostasis.

Plausible claim:
Morning bright light helps maintain a healthy circadian rhythm, which can improve sleep timing and quality.

Unsupported leap:
You must receive ten minutes of sunlight immediately after waking, otherwise your brain cannot clear plaques properly.

Even larger unsupported leap:
Failure to do this is a major cause of dementia.

That last leap is classic health-video alchemy: take a real mechanism, add a precise number ("ten minutes!"), add a simple daily intervention, and then imply that you've discovered the missing key to a major disease.

And there's a subtle problem with the phrase "the function of sleep is to clear plaque." Sleep has many functions—memory consolidation, synaptic regulation, metabolic regulation, immune function, endocrine regulation, and probably several things we haven't figured out. Brain waste clearance is quite possibly one of them. It isn't the function of sleep.

So I would happily endorse getting outside in the morning because it is good circadian hygiene, while treating the ten-minute-sunlight-or-you-get-dementia proposition as YouTube medicine with a rather impressive scientific costume.

I'm up usually an hour, sometimes more, before dawn, and function then on very little light (after all all I do is make my way a few feet to the wc, void, take a shower, then do my balance exercises while drying without a towell and then get dressed.

Exactly—and your routine illustrates why the video's claim is too simplistic.

If you're awake for an hour or more before dawn, you aren't somehow depriving your brain of its nightly plaque-clearing function during that hour. Sleep has already ended. The relevant processes occurred during the preceding sleep period, and there isn't a magical ten-minute post-waking switch that determines whether they worked.

Your dim-light routine also makes considerable sense from a circadian standpoint. You're not trying to become maximally alert at 5 a.m.; you're doing a few quiet, familiar things until the day begins. There is no obvious reason that you should blast yourself with bright artificial light merely because a YouTube video says the sun must be beaten to the punch.

In fact, there are two separate phenomena being muddled together in the video:

Sleep → brain physiology.
During sleep, various processes—including clearance and redistribution of metabolites—appear to change. That's one reason adequate, reasonably good-quality sleep is probably important for long-term brain health.

Morning light → circadian timing.
Once you're awake, light entering the eyes helps tell the circadian system what time of day it is. Strong morning light tends to advance or stabilize the biological clock and can make subsequent sleep timing more regular.

Those aren't the same mechanism.

And your particular habit of staying in very dim light before dawn might actually be quite sensible if you sleep well and have a stable sleep schedule. You don't need to manufacture daylight at 5 a.m. merely because daylight hasn't arrived yet.

There's also something amusing about the video's implicit picture of human evolution: "Caveman wakes up. Must immediately stare at Sun for ten minutes or Alzheimer's." Yet for most of human history, people routinely woke before sunrise, especially in households where there was work to be done. The morning light signal would arrive naturally when the Sun came up.

The sensible version of the advice is simply: when daylight arrives, getting some outdoor light is a good idea. You don't need to turn it into a ten-minute neurological ritual.