[Biology] Importance of sleeping

I went looking for an answer to a small question — does sleeping produce vitamins? — and ended up somewhere more interesting. The short answer is no, but the long answer is that sleep decides how well your body can use the vitamins it already has, and that the two stages people care about, REM and deep sleep, are doing completely different jobs.

Sleep doesn’t make vitamins

Nothing is synthesized because you’re asleep. The vitamins that your body does produce come from elsewhere: vitamin D from sunlight hitting your skin, vitamin K and biotin from gut bacteria working around the clock, niacin from tryptophan through a pathway that doesn’t care what time it is.

What sleep does instead is run the machinery that spends those nutrients. Protein synthesis needs B vitamins. Tissue repair needs vitamin C and zinc. Bone remodeling depends on how vitamin D and calcium are being handled. All of that is scheduled around sleep, and absorption and utilization degrade when sleep does. So the honest framing is less “sleep makes vitamins” and more “sleep is when your body actually gets to use them.”

What REM is for

Adults spend roughly a fifth to a quarter of the night in REM, and the REM periods get longer toward morning — which is exactly the part you cut when you sleep six hours instead of eight.

The clearest thing REM does is consolidate memory, especially the procedural kind: skills, pattern recognition, the sense that a problem you were stuck on yesterday is somehow easier today. If you spent the day grinding competitive programming problems or reading papers, REM is where that gets filed.

It also seems to strip the emotional charge off memories while keeping the memory itself. That’s the mechanism behind something feeling less awful the morning after, and it’s why chronic REM deprivation tracks with anxiety and depression.

The creativity claim is related. During REM the brain links concepts that don’t obviously belong together, which is a decent description of what an insight feels like from the inside.

And in infants REM can be half of total sleep, because it’s building neural circuits, synapses, and sensory processing from scratch. Adults are running the maintenance version of the same process — strengthening the connections that got used, weakening the ones that didn’t, so the network stays efficient instead of saturating.

Deep sleep is doing the other half

REM gets the attention, but N3 deep sleep is where the body gets repaired. Most growth hormone release happens there. So does muscle recovery, immune function, and glymphatic clearance — the brain flushing out metabolic waste, which is a genuinely strange and recent thing to know about.

A normal adult night breaks down roughly as:

Stage	Share of the night	What it’s for
Light (N1 + N2)	50–60%	transition, consolidation
Deep (N3)	15–25%	physical repair, growth hormone, immune function, brain waste clearance
REM	20–25%	learning, memory, creativity, emotional regulation, plasticity

So is more REM better?

No — the ratios are the point. Too little REM shows up as impaired learning and worse emotional regulation. Too little deep sleep shows up as a body that isn’t recovering and an immune system that isn’t keeping up. You want both, in something close to those proportions, which mostly means sleeping long enough that you don’t truncate the REM-heavy back half of the night.

How long, then?

Seven to nine hours, for a healthy adult. That number is better supported than most sleep advice, and a few things about it are worth knowing.

It’s a U-curve rather than “more is better.” Large population studies find worse outcomes at both ends — under about six hours and over about nine or ten both correlate with higher mortality and cardiovascular risk. The long end is probably mostly reverse causation, since being ill makes you sleep more, but either way there’s nothing to gain from chasing ten hours.

Individual variation is real and much smaller than people want it to be. Genuine short sleepers who do fine on six hours exist and carry rare mutations in genes like DEC2 and ADRB1 — roughly one in ten thousand people, not one in ten. Nearly everyone who believes they’re one of them is chronically deprived and has stopped noticing. That’s the cruel part of sleep debt: subjective alertness recovers, objective performance doesn’t.

Age moves the target. Teenagers actually need eight to ten hours and their circadian phase runs late, which is why early school start times are a fight nobody wins. Older adults often sleep in shorter blocks, but the underlying need doesn’t drop much — the sleep fragments rather than shrinks.

Duration and timing interact, which is where the stage breakdown above starts to matter. Because REM is concentrated in the back half of the night, cutting from eight hours to six doesn’t shave a quarter off each stage evenly — it disproportionately amputates REM. Deep sleep front-loads and is largely protected. That’s the mechanism behind short sleep hitting learning and emotional regulation harder than physical recovery.

Regularity may count for as much as total hours. Recent work suggests consistent sleep and wake times predict mortality at least as well as duration does, so seven hours on a fixed schedule beats averaging eight on a chaotic one.

Fix the room, not just the schedule

All of the above assumes you actually stay asleep, and the two things most likely to stop that are boring and physical rather than behavioural.

If you live anywhere with mosquitos, get a net. It is worth more than any amount of sleep-hygiene advice. A single mosquito doesn’t wake you up once — it pulls you out of deep sleep repeatedly across the night, and those arousals are short enough that you won’t remember them in the morning. You’ll just feel like eight hours didn’t work. A net costs almost nothing and removes the problem entirely, which is a better deal than repellent you have to reapply or a machine you have to remember to switch on.

Temperature is the other one. Core body temperature has to drop for sleep to start and stay down for deep sleep to hold, so a warm room fights you all night. This matters more than people in Indian cities like Bangalore or Hyderabad tend to assume — the weather is pleasant enough that the room never feels hot, just moderately warm, which is exactly warm enough to keep you in lighter stages without ever waking you enough to blame it. An air cooler is usually plenty in that climate; an AC if the humidity is bad. Cool and slightly under a blanket beats warm and uncovered.

The practical test is simple. If you wake up without an alarm at roughly the same time and don’t crash in the afternoon, you’re getting enough. Needing an alarm every single day means you’re running a deficit.

Sleep doesn’t manufacture anything. It just decides how much of what you already have actually gets used.