Tired but can't fall asleep — five most likely causes.

Cortisol is supposed to peak in the morning and trough in the evening. Stress flattens the curve — your evening cortisol stays elevated, and you can't enter the parasympathetic state required for sleep onset.^[1]

The fix isn't "relax." Cortisol responds to physical signals, not mental willpower. The body needs three things to lower cortisol enough for sleep:

1. **Dim light** — overhead lights below 100 lux for 90 minutes 2. **Cool body temperature** — a 1-2°C drop in core temperature is part of the sleep onset signal 3. **Slow breathing** — 6 breaths per minute for 4+ minutes activates the parasympathetic system

A 90-second box-breath at the bedroom door before you start the routine is more effective than 30 minutes of mental "winding down." The breathing technique: inhale 4 counts, hold 4, exhale 4, hold 4. Repeat for 90 seconds. Studies on heart rate variability show this measurably shifts autonomic balance — and HRV is the proxy that predicts sleep onset latency.

If your weekend bedtime is 90+ minutes later than your weekday bedtime, you're effectively flying east every Monday. The circadian system needs ~1 day to shift 1 hour of phase, so by Tuesday or Wednesday you're misaligned with your alarm.^[2]

This is "social jet lag" — first described by Roenneberg in 2006 and now one of the best-studied causes of weekday sleep onset problems. The data: 70% of working adults experience some social jet lag, and ~30% have shifts of 2+ hours every weekend.

Pull the weekend bedtime within 30 minutes of weekday bedtime. Wake time matters more than bedtime — anchor the wake side, even on weekends, even after a bad night. The wake-time anchor is what re-sets the circadian clock; bedtime drift will adjust on its own once wake-time is consistent.

If you can't or won't anchor weekend wake-time, accept that Sunday/Monday onset will be hard and don't catastrophise it as insomnia. It's a predictable cost of the schedule choice.

Covered in detail in our caffeine article — the short version is that caffeine's half-life is 5 hours on average and longer for slow metabolizers. A 2pm coffee at a 7-hour half-life leaves a meaningful residue at 11pm.^[3]

Tolerance hides this. You feel fine drinking coffee late. The caffeine still suppresses adenosine receptors, raising the arousal floor enough that sleep onset takes 30-60 extra minutes — and you might attribute that to "stress" or "racing mind" rather than the chemistry it is.

Test: cut all caffeine after noon for 5 nights and see if onset speeds up. Most people who think tolerance protects them see a measurable difference within a week. If you do, you've identified the cause. If you don't, move on to causes 1, 2, 4, or 5.

Bright light suppresses melatonin. The threshold is around 100 lux for blue-rich light. A bathroom overhead is 200-500 lux. A phone screen at full brightness held 30cm from your face is ~40 lux at the eye, which is below the threshold but compounds with overhead lighting.^[4]

The fix is the cheapest one in this list: dim every light to under 100 lux 90 minutes before bed. Warm-white bulbs help, but lumen reduction matters more than color temperature. The simplest intervention: install dimmer switches or use smart bulbs scheduled to dim at 9pm.

Screen content is more activating than the light itself. Doomscrolling at 100 nits is worse than a paper book at the same lux because the content keeps cortisol elevated. If you're not willing to give up screens, switch to lower-stimulation content (a slow-paced film, an e-reader) and reduce brightness to minimum.

Blue-light blocking glasses ("orange goggles") have decent evidence for melatonin preservation, but the benefit washes out if overall light intensity is still high. Dim first, filter second.

If you've spent enough nights in bed unable to sleep, your brain learns to associate the bed with wakefulness rather than sleep. The bed itself becomes a stimulus that prevents sleep.^[5]

This is the cause people most often miss because it doesn't feel like a "cause" — it feels like the problem itself. The diagnostic test: do you feel sleepy on the couch but instantly alert when you lie down? That's conditioned arousal.

Cognitive Behavioral Therapy for Insomnia (CBT-I) addresses this with stimulus control: only get into bed when sleepy, and if you're awake more than 20 minutes, get out of bed and do something boring under dim light until sleepy. Boring is the operative word — no phone, no TV, no scrolling.

This works better than melatonin, sleeping pills, or any supplement. It is the most evidence-backed insomnia treatment available, and it's free. The downside: it takes 2-3 weeks of strict adherence to re-condition the bed-sleep association. Most people quit at week one when nights get worse before they get better. That's the expected pattern; push through.

Apps like Sleep Reset and CBT-i Coach (free, US Department of Veterans Affairs) deliver the protocol without a therapist.