Are Your Days Helping or Hurting Your Health?

Sarah Bayliss
Jul 3
11 min read

You Run on Light and Dark

It’s not just your sleep. The light and dark you live by are quietly shaping your blood sugar, your heart and how well you recover, and most people don't know the full picture.

Our lives are almost the exact opposite of what our biology expects.

For the first ten thousand generations of human existence, we lived outdoors, bright light on our skin and in our eyes most of the day, real darkness at night lit only by fire and stars, food that changed with the seasons, movement, rest that followed the sun. That is the world our biology was built for, and still expects. Our genes haven’t meaningfully changed. But in just a few generations our environment has flipped. We now spend around 90% of our time indoors, under light too dim for the body to read as daytime, and then we light ourselves up at night with screens and bulbs our ancestors never knew. We’ve gone from a life lit by the sun to a life lit by technology, and nobody told our cells.

That gap, between the life our biology expects and the life we actually live, is the mismatch. And I’ve come to believe it sits quietly behind a great deal of how we feel, and how much metabolic resilience we hold: the capacity to make energy, handle pressure and recover well.

When your best starts to slip

You might notice it first as tiredness, sleep that no longer quite restores you. A mind that’s lost a little of its edge. A mood that’s flattened; weight that’s harder to shift than it used to be. Sometimes it shows up in your blood work, too: glucose, cholesterol or blood pressure beginning to drift in the wrong direction. Plenty of people never join the dots, and put it down to age, or stress, or simply how things are now.

It often creeps in through our forties and fifties, the decade when the body stops doing what it always used to do so easily, and the things that quietly self-corrected start to drift. It’s tempting to read that as the start of decline. More often it’s something more useful than that: your body communicating that something has shifted and wants attention.

Symptoms are messages, not verdicts, and this is a stage where there’s still a great deal you can do. This, really, is the decade that decides how the next thirty years feel: the years are either building you or eroding you, and you have far more say in which than you might think.

You’ve been turned down

Here’s a more useful way to picture it. You’ve been turned down. Like a dimmer switch eased towards the bottom, you’re not shining as bright as you’re built to; the whole system is running at a lower amplitude than it should. The good news is that a dimmer switch turns back up.

Most people are simply living out of tune and out of time, without realising the toll it quietly takes — on their metabolism, their energy, their mind and mood. I’m a nutritional therapist, and food matters enormously; I’ll come back to it. But after years in clinic, I’m convinced that for most people this is the layer to get right first. To see why, it helps to know what’s actually keeping time inside you.

The clock inside every cell

You’re a living organism, and like every living thing, you run on the rhythm of light and dark. Here’s how it works. Light and darkness reaching special cells in your eyes set a master clock in your brain, and that clock sends a time signal out to a second clock sitting inside almost every cell you have. Those cellular clocks decide when each organ does its job: when your pancreas releases insulin, when your blood pressure rises and falls, when your gut digests, and when your cells switch from working to repairing.

Each of those deserves attention, and repair is the one we most often forget. The day is for output. The night is downtime, and downtime is when the body repairs: clearing the day’s wear, resetting how it handles blood sugar, restoring what ran all day. Rest and repair, in the same window.

When the light and dark signals are right and consistent, those billions of clocks stay in step. The day runs the day; the night runs the repair. When the signals are wrong, too little light by day, too much at night, they drift apart. Your brain and liver still thinks it’s day, and the night’s repair gets cut short. Over time that doesn’t only show up as “bad sleep”. It shows up in the body: blood sugar that’s harder to hold steady, blood pressure that doesn’t dip overnight the way it should, a system slower to recover from what you ask of it.

Stress, the gut, what and when you eat — they all feed in too. But this timing sits underneath them, which is why I start here, and why I look at it before chasing any single symptom.

Light is a nutrient

Full-spectrum sunlight is more than a signal: it’s information and an essential nutrient, as real as anything on your plate. And so is darkness.

The full prescription isn’t only light, it’s light and dark, each at the right time: bright days, genuinely dark nights, the whole 24-hour cycle. Your body reads the absence of light at night as carefully as it reads its presence by day.

Most of us think of vitamin D and stop there when thinking about sunlight. That’s a sliver of it. Sunlight is a full spectrum, and we evolved needing all of it: the blue that tells your clock it’s daytime; the red and near-infrared that reach deep into the body and feed the mitochondria, the powerhouses that don’t only make energy but help run the cell itself, though energy is the heart of what they do; and, in sensible doses, the ultraviolet that releases nitric oxide from the skin (more on that, and your heart, shortly). More than half the light in real sunlight is infrared, and almost none of it reaches us indoors.

Because that’s the modern problem. Indoor light gives you a thin, mistimed sliver - bright and blue-rich in the evening when you should be winding down, with little of the red and infrared your cells thrive on. Our bulbs lack it; our windows filter it out. Professors Glen Jeffery and Robert Fosbury at UCL have shown how much those longer wavelengths matter to the way cells make energy. And it isn’t only energy, bright daytime light sharpens the waking brain, while dim indoor days quietly blunt it.

The trouble with bright nights

You’ve probably heard that screens at night are bad for sleep. Fewer people know how far it reaches. By day, blue light is exactly right — it tells the brain it’s daytime and sharpens focus. At night, the same light is the problem. Screens and LED bulbs are blue-rich, so late-evening light tells your brain daytime just as your body is trying to begin the night. It suppresses melatonin — and melatonin is far more than the hormone that makes you sleepy. It’s one of the body’s master antioxidants, the crew that works the overnight repair shift. Dim it night after night and you don’t simply lose sleep; you cut short the repair the night was for, and push your whole clock later. Which brings us to the part that doesn’t get said enough.

This reaches your heart and your blood sugar

Here’s what gets lost when all of this is filed under “sleep.” The same mistimed light and broken rhythm that disturb your nights are linked, in large studies, to the conditions people most want to stay ahead of especially in their forties and fifties and beyond: type 2 diabetes, raised blood pressure and heart disease.

It comes back to timing and light. Blood sugar and blood pressure run on the clock, your body handles the same meal differently at 8am and 10pm, and insulin works better earlier in the day. Disturb the clock and that control slips. In controlled studies, a single night spent sleeping in even modest light raised next-morning insulin resistance and heart rate, and evening blue light has been shown to push blood sugar higher.

It also shows up at scale: in a study of nearly 89,000 adults wearing light sensors, those with the brightest nights had around a 40% higher risk of dying from cardiometabolic disease, and over the following decade higher rates of heart failure, heart attack, stroke and type 2 diabetes. These are associations, and other things play their part, but they’re large, they’re consistent, and they hold even after accounting for diet, exercise, smoking and genetics.

Now the hopeful half, because this cuts both ways: the right light by day actively protects the same systems. Sensible sunlight on the skin releases nitric oxide, which relaxes blood vessels and lowers blood pressure, a cardiovascular benefit entirely separate from vitamin D, shown by Professor Richard Weller at the University of Edinburgh. And while too much sun burns and skin cancer is real, too little carries its own cost: in large studies, people who avoid the sun don’t live longer, they die sooner, with one long Swedish cohort putting the size of that risk on a par with smoking.

The message was never “no sun”. It’s the right dose, regular, sensible, unburnt, matched to your own skin, the daily exposure your heart and metabolism are built for.

So this isn’t only a warning; it’s leverage. The light and dark you live by are among the few things shaping your cardiometabolic health that are free, daily, and genuinely within your control.

Food keeps time too

Food is a timekeeper too. When you eat sets the clocks in your gut, your liver, your pancreas. Eat in tune with daylight and those organs keep good time; eat late into the night and they fall out of step with the clock in your head. For most of human history we also ate seasonally, different foods at different points in the year, which was itself a signal of where we were in the cycle. Today we eat the same things year-round, at all hours, under bright light. So with food, timing matters as much as quality: meals that keep step with daylight work with your biology; the same meal at midnight works against it. Food is powerful, and most powerful when it is seasonal, from nature and keeps time with your clock. Food is light, captured sunlight, stored by plants and passed to your cells to function optimally.

A kind of cellular jet lag

If you want to feel what living out of sync actually does, think of jet lag. Fly across enough time zones and every system in you lags behind, awake when you should be asleep, hungry at the wrong hours, foggy, low, off.

Now imagine a milder version happening quietly, every single day, without ever leaving home: your internal clocks and your environment never quite agreeing. That is the state many of us are in. A kind of cellular jet lag and, like jet lag, something the body can recover from once the signals come back into line.

Why this matters to me and my work

Here’s why this matters so much to me. My focus is optimising metabolic resilience - good energy, balanced blood sugar, managing stress in a way the body makes and regulates energy under pressure.

Light and dark reach further into all of that than most people expect: they set the clock, feed the mitochondria, help steady blood sugar, support the nitric oxide that protects the heart, and shape the dopamine and serotonin rhythms behind drive and mood.

That doesn’t make light the answer to everything, nutrition, stress, the gut and the rest each do real work, and most people need changes at several levels. But it’s the first lever to get right, and the one most often dismissed or done badly as dismissed as not that important.

So with a new client, one of the first things I want to understand has nothing to do with their plate. It’s their light: how much daylight they really get, and how dark their nights really are and what the rhythm of their day looks like.

How I work: start at the foundation

This is the heart of how I work, and the model I’ve built my practice around. You don’t address all of it at once, and you don’t start in the middle. There’s an order. You begin at the foundation - light, dark and rhythm, because it sets everything downstream.

Then you settle the stress response and the nervous system. Then the gut, and what and when you eat. And as that base comes back into sync, the things people actually came for - energy, sleep, mood, hormones, metabolism start to come good.

They were the symptoms all along, not the cause. Because resilience is biological = your body’s capacity to absorb stress, recover, and stay well over time with metabolic resilience, the way you make and regulate energy, right at its heart. You build it from the ground up. And the ground is light and dark.

If you change a few things this week

The best part is that the most powerful lever you have is free, and your body responds quickly. Consistency really matters. If you change nothing else this week, do this every day from now on:

Get outside into morning light within 30mins for 20-60 mins, ten minutes, longer when it’s grey. This one matters most.
Keep a steady wake and sleep time. Consistency is what anchors the whole clock. The system is unsure of itself so daily inputs really do matter.
Take your daylight in the middle of the day too
Dim your evenings lamps rather than overhead lights, block as much artificial blue from your environment as you can, consider changing your indoor light to circadian friendly solutions / incandescent bulbs
Set a screen curfew, ideally 2 hours to 90 mins before bed.
Eat a little earlier, and protect the dark when you sleep.
Don't skip breakfast.

None of it is hard, but requires consistency, and daily practices that are non-negotiable to protect your health, vitality mind and mood.

All of it works with your biology instead of against it.

This is what I do:

I help people come back into sync with their biology, even in a world that pulls the other way, beginning with light, working through in order, until the whole system runs better. Because light and rhythm don’t sit in one corner of your health. They run underneath all of it: your metabolism and blood sugar, your heart, your stress and resilience, your gut, your immune system, your focus and mood. It’s the foundation of my clinical work, and the talk I bring to teams and organisations, We’re Living in Biological Darkness.

If you’re in the decade where this starts to count, don’t file it under “one day”. It’s the highest-leverage place to begin, and you can start tomorrow morning. I’ll continue to share the science and the how, one practical piece at a time, because once you’ve seen it, you can’t unsee it: you run on light and dark.

The research behind this

A few of the researchers and studies referred to above, for anyone who wants to read further. The fuller evidence, with each claim tiered by how strong it is, sits in my light evidence base.

The body clock — the 24-hour clock that organises our biology won the 2017 Nobel Prize in Physiology or Medicine (Jeffrey C. Hall, Michael Rosbash and Michael W. Young).
Red and near-infrared light and the mitochondria — Professors Glen Jeffery and Robert Fosbury, UCL. Kaynezhad P, Fosbury R, Hogg C, Tachtsidis I, Sivaprasad S, Jeffery G. Near-infrared spectroscopy reveals instability in retinal mitochondrial metabolism and haemodynamics with blue light exposure at environmental levels.Journal of Biophotonics, 2022.
Sunlight, nitric oxide and blood pressure — Professor Richard Weller, University of Edinburgh. Liu D, Fernandez BO, Hamilton A, et al. UVA irradiation of human skin vasodilates arterial vasculature and lowers blood pressure independently of nitric oxide synthase. Journal of Investigative Dermatology, 2014.
Sun avoidance and mortality (the Swedish cohort) — Lindqvist PG, Epstein E, Nielsen K, et al. Avoidance of sun exposure as a risk factor for major causes of death. Journal of Internal Medicine, 2016.
Light at night, mortality and cardiovascular disease (nearly 89,000 people) — Windred DP, Burns AC, Lane JM, et al. Brighter nights and darker days predict higher mortality risk. PNAS, 2024; and Light Exposure at Night and Cardiovascular Disease Incidence. JAMA Network Open, 2025.
Light at night and type 2 diabetes — Windred DP, Burns AC, Rutter MK, et al. Personal light exposure patterns and incidence of type
2 diabetes. Lancet Regional Health – Europe, 2024.
Light during sleep and next-morning insulin resistance — Mason IC, Grimaldi D, Reid KJ, et al. Light exposure during sleep impairs cardiometabolic function. PNAS, 2022.

Sarah x