Trouble sleeping at night usually comes down to one or more fixable problems: your brain is too alert, your body clock is confused, or something in your environment or health is working against you. Adults need 7 to 9 hours of sleep, but hitting that number requires your biology, habits, and surroundings to cooperate. Here’s what’s most likely keeping you up and what you can do about each one.
Your Brain Won’t Quiet Down
The single most common reason people can’t fall asleep is cognitive arousal, the clinical term for a mind that won’t stop running. Worry, planning, replaying the day, rehearsing tomorrow’s conversation: these patterns of rumination tend to peak in the late evening and night, right when you need your brain to wind down. People with high levels of this nighttime mental activity take roughly an hour to fall asleep and sleep fewer than six hours total. This isn’t just an annoyance. It triggers a real physiological response, raising your heart rate and keeping your stress hormone levels elevated in a way that blocks the transition into sleep.
Stress hormones play a direct role here. Your body normally dials cortisol down to its lowest point around midnight, then gradually ramps it back up toward morning. When your stress response system is overactive, cortisol stays elevated at night, fragmenting your sleep cycles and shortening total sleep time. If you’ve noticed that you can sleep fine on weekends or vacations but struggle on work nights, this pattern is a strong clue that stress and mental arousal are the primary culprits.
Light Is Resetting Your Internal Clock
Your body decides when to feel sleepy largely by producing melatonin, a hormone made by a small gland in your brain. The process is tightly controlled by light. When your eyes detect darkness, a chain of chemical signals ramps up melatonin production. When your eyes detect light, that chain is shut down almost immediately by suppressing the key enzyme that drives the whole process.
Blue light, the wavelength dominant in phone screens, laptops, and LED bulbs, is particularly disruptive. In a Harvard experiment, 6.5 hours of blue light exposure suppressed melatonin for about twice as long as green light of the same brightness and shifted the body’s internal clock by 3 hours compared to 1.5 hours for green light. That means scrolling your phone in bed isn’t just a minor bad habit. It can push your natural sleep window hours later than it should be, making it physically difficult to fall asleep at a normal time even if you’re tired.
If you’re someone who feels wide awake at 11 p.m. but exhausted by morning, excessive evening light exposure is one of the first things to address. Dimming overhead lights after sunset and putting screens away at least an hour before bed gives your melatonin production a chance to kick in on schedule.
Caffeine Lasts Longer Than You Think
Caffeine works by blocking the receptors in your brain that detect a sleep-promoting chemical called adenosine. Adenosine builds up naturally throughout the day, creating increasing pressure to sleep. Caffeine sits in those receptors and prevents you from feeling that pressure, which is great at 8 a.m. but problematic at 10 p.m.
The half-life of caffeine in adults is 2.5 to 4.5 hours, meaning half the caffeine from your afternoon coffee is still active in your brain up to 4.5 hours later. A large coffee at 3 p.m. could still have a quarter of its caffeine circulating at 10 p.m. The result is longer time to fall asleep, more waking during the night, and lighter sleep overall. If you’re sensitive to caffeine, a cutoff time of noon or 1 p.m. is more realistic than the commonly suggested “no coffee after 2 p.m.”
Your Bedroom May Be Too Warm
Falling asleep requires a slight drop in core body temperature. If your room is too warm, your body can’t cool down efficiently, and you’ll lie there feeling restless without understanding why. Sleep specialists at the Cleveland Clinic recommend keeping your bedroom between 60 and 67°F (15 to 19°C). This temperature range also helps stabilize REM sleep, the stage most important for memory and emotional processing. If you tend to kick off the covers or flip your pillow looking for the cool side, your room is probably too warm.
Restless Legs and Iron Levels
If your inability to sleep involves an uncomfortable urge to move your legs, or a crawling, tingling sensation that gets worse when you lie down, you may have restless leg syndrome. One of the most overlooked causes is low iron, specifically low iron stores in the brain. What makes this tricky is that your blood iron levels can look perfectly normal on a standard test while your brain’s iron supply is depleted.
The key test to ask for is ferritin, which measures stored iron. Experts recommend treatment when ferritin is at or below 50 mcg/L, and iron supplements relieve symptoms substantially in about half of people at those levels. Improvement isn’t instant, though. It typically takes about a month for iron levels to rise enough that symptoms start to fade. Harvard Health notes that many people with iron deficiency and restless legs are never tested for this connection, so it’s worth raising with your doctor if the symptom fits.
Sleep Apnea You Might Not Know About
Some people with obstructive sleep apnea have no obvious symptoms. They don’t snore loudly or gasp for air. They simply don’t feel rested after sleeping, feel sluggish during the day, or wake up repeatedly without knowing why. Sleep apnea happens when the airway partially or fully collapses during sleep, reducing or stopping airflow for at least 10 seconds at a time. To be diagnosed, these episodes need to occur at least 5 times per hour.
Severity is measured on a scale: 5 to 15 episodes per hour is mild, 15 to 30 is moderate, and more than 30 is severe. The condition is more common in people who are overweight, but it can affect anyone, including thin, young adults. If you sleep enough hours but still feel exhausted, or if a partner has noticed pauses in your breathing, a sleep study can rule this in or out definitively.
Aging Changes Sleep Structure
If you used to sleep well and gradually started struggling, age itself may be part of the picture. As you get older, your sleep architecture shifts. You spend less time in deep sleep (the physically restorative stage) and less time in REM sleep (the mentally restorative stage). Sleep becomes more fragmented, lighter, and shorter. Your internal clock also loses some of its amplitude, which is why many older adults feel sleepy earlier in the evening and wake up earlier in the morning.
These changes are normal and don’t automatically mean something is wrong. The recommended sleep duration for adults over 65 is 7 to 8 hours, slightly less than the 7 to 9 hours recommended for younger adults. That said, poor sleep in older adults often has a treatable cause layered on top of normal aging, whether it’s medication side effects, pain, or an undiagnosed sleep disorder.
When It Becomes Clinical Insomnia
Not every bad night is insomnia. The clinical threshold is specific: difficulty falling or staying asleep at least three nights per week for three months or more, despite having adequate opportunity to sleep. If your sleep problems are newer than three months, they’re classified as short-term insomnia, which often resolves on its own or with simple changes to habits and environment.
Chronic insomnia that meets the three-month mark tends to be self-reinforcing. You start associating your bed with frustration, your anxiety about sleep makes sleep harder, and compensating behaviors like sleeping in or napping can further disrupt your body clock. The most effective treatment for this cycle is a structured behavioral approach that retrains your brain’s association between bed and sleep. It works better than medication for long-term outcomes and doesn’t carry the risks of dependence.
Practical Changes That Make the Biggest Difference
If you’re not sure what’s keeping you up, start with the factors that affect the most people. Keep your bedroom cool, between 60 and 67°F. Reduce light exposure in the evening, especially from screens, for at least an hour before bed. Cut caffeine by early afternoon. These three changes alone resolve mild sleep problems for many people.
For racing thoughts, the goal is to move your processing time earlier. Spend 10 to 15 minutes before bed writing down what’s on your mind or planning the next day, so your brain doesn’t try to do that work after lights out. Physical activity during the day also lowers nighttime cortisol and reduces the time it takes to fall asleep, though intense exercise within two to three hours of bedtime can have the opposite effect.
If you’ve addressed the obvious factors and still can’t sleep after several weeks, the issue may be medical. Low iron, sleep apnea, thyroid imbalances, and certain medications can all quietly sabotage sleep without causing symptoms you’d connect to the problem. A conversation with your doctor, ideally with a sleep diary showing your patterns, is the logical next step.