I've never been a morning exerciser. My body rebels against early workouts with a sullenness that lasts until noon. But everything I read insisted morning exercise improved sleep. So I spent six months experimenting with workout timing, tracking sleep with and without exercise at different times of day. My data contradicted the conventional wisdom—and subsequent research helped me understand why. Morning exercise absolutely can improve sleep, but so can afternoon and evening exercise, provided you leave adequate recovery time before bed. The relationship between exercise and sleep is powerful, but the timing prescriptions in most wellness articles lack nuance. The real mechanism involves body temperature elevation: exercise raises core temperature, and the subsequent cooldown mimics the natural temperature drop that precedes sleep onset. This temperature drop is the key, not the time of day. Vigorous exercise within 2-3 hours of bedtime can interfere with sleep for some people. Post-exercise adrenaline and elevated heart rate create physiological arousal incompatible with falling asleep quickly. However, this effect varies enormously by individual. Some people fall asleep easily after evening exercise; others are wired for hours. Understanding your personal response matters more than following generic timing rules. Moderate aerobic exercise consistently shows sleep improvements in research studies—probably because moderate exercise doesn't trigger the arousal that intense exercise does. Activities like walking, light cycling, swimming, or yoga produce the temperature benefits without the arousal effects. For most people, moderate exercise at virtually any time of day improves subsequent sleep quality, with the notable exception of immediately before bed. Resistance training may be equally important for sleep. Heavy strength training creates significant muscular repair requirements that increase deep NREM sleep drive. Studies comparing aerobic-only exercise with combined aerobic and resistance training show that the combined approach produces better sleep outcomes. This makes sense mechanistically: muscular recovery is metabolically demanding and drives sleep pressure that facilitates falling asleep. Consistency of exercise timing matters as much as the timing itself. People who exercise at unpredictable times often show worse sleep than those who exercise regularly but at unconventional hours. The body adapts to regular exercise timing, anticipating recovery demands and adjusting sleep architecture accordingly. Random schedule exercise prevents this adaptation, leaving the body uncertain about when to expect physical demands. Outdoor exercise provides circadian benefits beyond the exercise itself. Morning outdoor light exposure anchors circadian timing, making evening sleep onset easier regardless of the exercise effects. Afternoon outdoor exercise provides beneficial evening bright light exposure that can shift circadian timing later—problematic if you're already naturally a night owl, but potentially beneficial if you're trying to advance your sleep schedule earlier. The optimal approach: exercise at whatever time you can sustain consistently, prefer afternoon or evening for intense exercise if your schedule allows, avoid intense workouts within 90 minutes of bedtime if you're sleep-onset impaired, and get some outdoor exercise for circadian anchoring. These principles accommodate most schedules and produce better sleep than rigid adherence to morning-only rules that most people eventually abandon.