You avoid screens at night and still can't sleep. Your genes may be why.

Your CLOCK gene is the master regulator of your circadian rhythm. It tells your brain when to release melatonin, when to raise cortisol, when to feel alert, and when to feel tired. It does this by responding to light signals, physical activity timing, and meal timing. Think of it as the conductor of a 24-hour biological orchestra.

The CLOCK 3111T/C variant, carried by roughly 30 to 50% of people, alters how the gene product functions in the suprachiasmatic nucleus, your brain’s light-sensing hub. If you carry this variant, your circadian clock is less efficient at responding to light cues, and melatonin onset is delayed. This means evening light, especially from screens, has an outsized effect on your sleep timing. Where a person with the common variant might be slightly suppressed by blue light at 9 PM, you may experience a full two to three hour delay in melatonin release.

You probably notice this as: you feel alert much later than you expect, even when you’re lying in bed trying to sleep. Your “natural” bedtime is later than most people’s. Bright light in the evening keeps you wired, not just mildly alert. You may have a history of being a night owl, and “fixing” your sleep schedule has never quite worked because your clock is genuinely set differently.

PER3 is one of your body’s internal period genes; it works alongside CLOCK to regulate your circadian rhythm. But it does something else critical: it influences how quickly you accumulate sleep pressure, and how well your brain performs when you’re sleep-restricted. The gene exists in two main forms: a 4-repeat version and a 5-repeat version, based on the number of amino acid repeats in the gene sequence.

If you carry the 5-repeat variant (found in roughly 10 to 25% of people with European ancestry), your brain builds sleep pressure more slowly. You may feel like you need less sleep than people around you, or that you’re “wired” even when objectively tired. The flip side: when you do become sleep-restricted, your cognitive performance drops more sharply than it does in 4-repeat carriers. Screens exploit this; they keep you activated while your sleep pressure is still building slowly.

You probably experience this as: you can “push through” tiredness for hours after you should be asleep. You don’t feel the cumulative fog of sleep debt right away; it hits you suddenly after several nights of disruption. Screen use doesn’t make you feel tired the way it does for others. You feel energized or anxious instead. By the time you realize you’re truly sleep-deprived, the cognitive damage has accumulated.

Adenosine is the molecule that builds up throughout your waking day and creates sleep pressure, the growing drive to sleep. Your ADORA2A gene codes for the adenosine A2A receptor on your brain cells, the docking station where adenosine signals “time to sleep.” Caffeine works by blocking this receptor, which is why it keeps you alert. But the ADORA2A gene comes in variants that change how sensitive your receptors are to adenosine in the first place.

The C/C variant at rs5751876, found in roughly 10 to 15% of the population, produces a version of the receptor that’s less responsive to adenosine. If you carry this variant, your brain is naturally less sensitive to sleep pressure signals, and caffeine hits you harder and lasts longer. A cup of coffee at 2 PM stays in your system longer than it should, and the adenosine-blocking effect is more pronounced. But here’s the deeper issue: screens trigger caffeine-like neurochemical states (dopamine, norepinephrine) that mimic caffeine’s effect on the adenosine system. Your already-blunted sleep pressure signal gets further suppressed.

You probably notice this as: caffeine affects you much more than it affects friends who drink the same amount. A single cup of coffee can disrupt your sleep 10 hours later. After an evening of screens, you don’t feel sleepy for hours, even though you’re objectively tired. Your sleep pressure just doesn’t build the way it should.

SLC6A4 codes for the serotonin transporter, the protein that recycles serotonin back into nerve endings after it’s been released. Serotonin is a daytime neurotransmitter that creates mood, motivation, and social engagement. At night, serotonin gets converted into melatonin, the sleep hormone. If you can’t recycle serotonin efficiently, this conversion gets blocked, and melatonin production suffers. The 5-HTTLPR short allele is a structural variant in this gene that reduces the transporter’s efficiency.

Roughly 40% of people with European ancestry carry at least one short allele. If you carry the short form, your serotonin stays in the synaptic gap longer, which sounds good but actually impairs the circadian conversion of serotonin to melatonin at night. You end up with a brain that’s still somewhat activated at bedtime, and insufficient melatonin to initiate sleep architecture. Screens are especially disruptive because they keep dopamine and serotonin elevated; your brain never gets the signal that daytime has ended.

You probably experience this as: your sleep doesn’t feel restorative, even when you sleep 8 hours. You wake frequently or feel like you never quite fell into deep sleep. You may have a history of depression or anxiety that responds well to SSRIs, or a family history of these conditions. Evening screen use leaves you emotionally activated as well as mentally activated, making it harder to wind down.

COMT breaks down catecholamines, the stress and reward neurochemicals: dopamine, norepinephrine, and epinephrine. These keep you alert, motivated, and engaged. In the evening, they need to decline so your parasympathetic nervous system can activate and sleep can begin. Your COMT gene comes in two main variants: the Val allele (common, fast) and the Met allele (slower). The Val158Met polymorphism is the most studied; people homozygous for the slow Met allele make up roughly 25% of the population.

If you carry the slow Met/Met genotype, you clear dopamine and norepinephrine slowly. Your stress hormones stay elevated longer after a stressor, and your reward system stays activated longer after screen-based novelty and engagement. Checking email, scrolling social media, or watching videos triggers dopamine and norepinephrine release; if you metabolize these slowly, your brain stays in “on” mode long after you set the phone down. Your cortisol and adrenaline don’t downregulate the way they should before bed.

You probably experience this as: you feel wired or anxious in the evening even when you’re objectively not stressed. Your mind races when you try to sleep. You’re sensitive to stimulants like caffeine and screen novelty. Relaxation techniques help temporarily, but don’t address the underlying slow clearance of stress hormones. You may have a history of anxiety or ADHD.

CYP1A2 is the liver enzyme that metabolizes caffeine. It breaks down caffeine into inactive compounds so your body can excrete it. The speed at which this happens is largely determined by your CYP1A2 genotype. The *1F allele (associated with the slow phenotype) is carried by roughly 50% of the population, making slow caffeine metabolism very common. Some people are fast metabolizers who can drink coffee at 5 PM and fall asleep by 10; others need a hard cutoff at 10 AM.

If you carry the slow metabolizer allele (*1F), your body clears caffeine at roughly half the rate of fast metabolizers. Caffeine consumed at 2 PM can still be 50% present in your bloodstream at 10 PM, suppressing slow-wave sleep and REM sleep. The problem compounds with screens: screens suppress melatonin and elevate cortisol on their own, and if caffeine is still on board, the nervous system never gets a chance to fully downregulate. A morning coffee plus afternoon tea plus evening screen exposure creates a perfect storm.

You probably notice this as: you’re extremely sensitive to caffeine timing. Even a small amount consumed after noon disrupts your sleep that night. Other people can have coffee in the afternoon and sleep fine. You may have tried cutting caffeine entirely and found it helps, but you’ve never understood why you react so much more strongly than others.