Why can't I sleep? The most common causes and what to fix first

The question "why can't I sleep?" sounds simple but contains at least six different problems with six different answers. Someone who can't fall asleep because their mind is running has a different root cause than someone who falls asleep fine but wakes at 3 a.m. and can't get back down. Both are different from the person who sleeps eight hours and still feels exhausted, or the person who can sleep but only if they go to bed at 2 a.m. The category called "sleep problems" is actually a collection of distinct patterns, and the intervention that helps one often doesn't touch the others.

The useful first question isn't "how do I sleep better?" It's "which specific part of sleep isn't working, and why?"

The four main patterns

Sleep problems tend to cluster around four patterns. Identifying which one applies is the fastest way to narrow the causes.

Difficulty falling asleep (sleep onset insomnia) means lying awake for 20 minutes or more after going to bed, with the mind active and rest not arriving. The cause is usually an aroused nervous system that hasn't had time or conditions to downshift. Late caffeine, bright light close to bed, stress-activated cortisol, and an irregular sleep schedule are the most common drivers.

Waking in the middle of the night and struggling to return to sleep (sleep maintenance insomnia) means falling asleep reasonably well but surfacing at 1, 2, or 3 a.m. and lying awake for 30 minutes or more. The causes here are different: blood sugar dips, alcohol (which disrupts sleep architecture in the second half of the night), cortisol rhythm dysregulation, and magnesium insufficiency all contribute. This pattern is covered in detail in the waking up at 3 a.m. post.

Waking too early means surfacing at 4 or 5 a.m. feeling done despite not having enough sleep. This pattern is often associated with depression, high morning cortisol, and disrupted circadian rhythm. It's less common than the other two and often requires a different approach.

Sleeping enough hours but feeling unrestored means consistently spending seven or eight hours in bed but waking feeling as though you didn't sleep. This usually points to sleep quality rather than duration: fragmented sleep architecture, disrupted deep sleep or REM, sleep apnea, or alcohol use the night before are common causes.

Caffeine timing

Caffeine is the most widely used psychoactive substance in the world, and its interaction with sleep is the most underestimated driver of sleep problems in otherwise healthy adults. The half-life of caffeine is approximately five to six hours, with meaningful individual variation based on genetics and liver enzyme activity. A fast metabolizer will process a 200 mg coffee by early afternoon. A slow metabolizer may have half of it still active at midnight.

At practical terms: a 3 p.m. coffee for a slow metabolizer has the equivalent of one espresso's worth of adenosine-blocking activity still running at 9 p.m. The person lying awake thinking "I can't sleep" is often, at the biological level, a person who drank coffee too late and is now experiencing partially suppressed sleep pressure.

The fix is a caffeine cutoff, and the right time depends on individual metabolism. A noon cutoff works for most people. Sensitive metabolizers sometimes need 10 a.m. People who drink caffeine into the mid-afternoon and wonder why they're restless at 11 p.m. are experiencing a solved problem.

The secondary effect worth noting: late caffeine doesn't just delay sleep onset. It degrades sleep architecture even when the person does fall asleep. Light sleep replaces deep sleep in the first part of the night, and the restorative value of the sleep that does occur is lower. This is why someone who "sleeps 8 hours after an afternoon coffee" often wakes feeling less restored than expected.

Light and melatonin suppression

The second most common cause of sleep onset difficulty in modern life is blue-spectrum light exposure in the hours before bed. The photoreceptors that drive melatonin suppression are specifically sensitive to the wavelengths that phones, laptops, and LED lighting emit. Two hours of bright screen exposure in the evening has been shown to delay melatonin onset by up to 90 minutes.

This means that someone who goes to bed at 11 p.m. after scrolling until 10:45 is essentially telling their circadian system that it's 9:15 p.m. The biological drive for sleep hasn't arrived. Lying in bed feeling awake is the expected result.

The fix is light management in the 60 to 90 minutes before bed. Dim overhead lights and shift to warm-toned lamps. Reduce screen brightness and enable night mode. The last 30 minutes without screens is the most impactful window, but even modest dimming earlier produces measurable changes in melatonin onset. Amber-tinted glasses that block blue wavelengths work when screen avoidance isn't practical.

Cortisol and the stress response

The HPA axis manages cortisol throughout the day, with cortisol naturally highest in the morning and lowest in the late evening. When the stress response is running at an elevated baseline (due to chronic work stress, emotional load, poor sleep compounding itself, or nutritional gaps like magnesium insufficiency), cortisol doesn't taper as cleanly in the evening as it should. Elevated evening cortisol is a consistent driver of both sleep onset difficulty and middle-of-the-night waking.

This is also the mechanism behind the experience of "tired but wired": the body is fatigued but the arousal system is still running. The subjective sensation is exhaustion that doesn't translate into sleep readiness, because the neurological brake that usually brings wakefulness down isn't engaging cleanly.

Magnesium glycinate addresses one piece of this: adequate magnesium helps the HPA axis regulate more cleanly and supports NMDA receptor function that keeps evening neuronal excitability from running too high. The magnesium for anxiety post covers the cortisol-stress mechanism in detail, and the magnesium glycinate for sleep post covers the direct sleep application.

Lifestyle inputs matter here too. Aerobic exercise earlier in the day reduces the cortisol baseline over time. Consistent sleep and wake times anchor the circadian rhythm, which helps cortisol taper on schedule. Time away from screens and difficult decisions in the final hour of the day gives the system fewer reasons to stay activated.

Irregular sleep timing

The body's circadian system works best when sleep and wake times are consistent. Irregular timing, sleeping two hours later on weekends than weekdays, for instance, produces what chronobiologists call social jet lag: the circadian clock is continuously being asked to shift, and it never fully adapts to any single schedule.

The result is that Sunday-night sleep onset is often significantly harder than weekday sleep, not because of stress or caffeine, but because the biological clock is two hours behind where the schedule is asking it to be. The Sunday scaries often have a sleep timing component alongside the psychological one.

The most impactful single sleep hygiene change for most people with irregular schedules is a consistent wake time. Going to bed at varying times is less disruptive than waking at varying times, because the morning light anchor is what sets the circadian clock for the following cycle. A consistent 7 a.m. wake time, even on weekends, synchronizes the clock and makes the evening sleep onset progressively earlier and easier over one to two weeks.

What supplements address

Supplements are a downstream intervention for sleep, not a root cause solution. But they address specific mechanisms that diet and lifestyle don't always fully cover.

Magnesium glycinate at 200 to 400 mg elemental nightly supports HPA axis regulation and NMDA function, both of which contribute to the brain's ability to downshift in the evening. It's the most broadly applicable sleep supplement because the mechanisms it addresses are the ones most commonly underpowered by modern diets and high-stress lifestyles.

L-theanine at 200 mg in the wind-down window reduces the subjective sense of pre-sleep mental activation without sedating. It shifts brain wave patterns toward the alpha range associated with relaxed wakefulness and eases the transition into sleep onset. It works particularly well for the "tired but wired" pattern.

KAEVO Unwind combines magnesium glycinate at the research-backed dose for the evening recovery routine. The Night Reset bundle adds KAEVO Night for nights where additional support is needed alongside the mineral foundation.

The evening wind-down routine post covers how supplements fit within the broader 90-minute window that shapes sleep quality more than any single intervention.

What about sleep apnea

Sleep apnea is worth naming specifically because it's a common and underdiagnosed cause of poor sleep quality and daytime fatigue that the behavioral and supplement interventions above don't address. In obstructive sleep apnea, the airway partially or fully collapses during sleep, interrupting breathing and triggering brief arousals throughout the night. Many people with sleep apnea don't experience these arousals as waking; they simply feel chronically tired, have difficulty with concentration, and sometimes wake with headaches.

The signals that suggest sleep apnea rather than the more common sleep-hygiene-related issues: snoring (especially loud or irregular snoring), being told you stop breathing during sleep, waking frequently needing to urinate, morning headaches, and daytime fatigue that persists despite spending adequate time in bed. The daytime sleepiness with sleep apnea is usually more severe and more consistent than the fatigue from poor sleep hygiene.

If these patterns apply, the appropriate path is a sleep study (now available as an at-home overnight monitor for most people) rather than adjusting caffeine timing or adding magnesium. Sleep apnea is a physical obstruction issue and is treated with continuous positive airway pressure (CPAP), positional changes, weight management in relevant cases, or dental devices. It's outside the scope of what any supplement or behavioral routine can meaningfully address.

For everyone without sleep apnea, the patterns described in this piece cover the causes of the vast majority of sleep complaints in otherwise healthy adults.

Where to start

The most efficient path through a sleep problem is to identify which pattern is present first, then address the most likely cause before adding supplements. Someone with sleep onset difficulty who drinks a daily 4 p.m. coffee should cut the caffeine before adding L-theanine. Someone with middle-of-the-night waking who drinks alcohol regularly should reduce evening alcohol before trying magnesium. The supplement is more effective when the behavioral drivers are handled first.

Once the behavioral inputs are reasonable, supplements close the remaining gaps. The combination of consistent sleep timing, appropriate light management in the evening, a caffeine cutoff before early afternoon, and a nightly magnesium glycinate dose addresses the four most common drivers of poor sleep in otherwise healthy adults. Most people who do all four notice a meaningful shift within two to three weeks.

The short version

Most sleep problems have a specific cause. Sleep onset difficulty usually traces to caffeine timing, evening light, or an elevated stress response. Middle-of-the-night waking usually traces to cortisol rhythm, blood sugar, or alcohol. Early waking and unrestored sleep have their own drivers. The fix starts with identifying which pattern is present, then addressing the most likely cause in order. Supplements support the system once behavioral inputs are handled. Magnesium glycinate is the most broadly applicable evening supplement because the mechanisms it addresses are the ones most commonly underpowered. A few practical notes before applying the framework. Prescription sleep medications are outside the scope of this post, but they're worth knowing about for the subset of people who need them. For acute insomnia (a few bad nights during a stressful period), the behavioral approach above usually resolves the pattern within one to two weeks without medication. For chronic insomnia (three or more months of significant sleep difficulty affecting daily function), a conversation with a doctor is warranted regardless of what behavioral and supplement approaches are tried first.

The boring but accurate answer is that most sleep problems are solvable, and the solution is usually a few consistent changes rather than a dramatic intervention.