Why Does Exercise Make You Feel Good?

After a run that almost broke you, something shifts.

Not immediately — for the first twenty minutes, you might just feel like you’re dying. But somewhere after that, for some people, something else happens. The effort becomes easier to bear. Sometimes it becomes almost pleasant. By the end, there’s something that resembles euphoria.

This is called runner’s high. People also report versions of it after cycling, swimming, long hikes, and other sustained aerobic exercise.

Everyone knows the explanation: endorphins.

Everyone is mostly wrong.

The Endorphin Problem

Endorphins — endogenous opioids — are genuinely released during strenuous exercise. That part is true. They act on the same receptors as morphine and heroin, producing pain relief and, at sufficient concentrations, euphoria.

The problem is simple: endorphins don’t easily cross the blood-brain barrier.

The blood-brain barrier is a highly selective filter that prevents most large molecules from entering the brain from the bloodstream. Most endorphins produced in the body during exercise can’t get through in significant quantities. The high they’d produce requires brain access — which they mostly don’t have.

There are endorphins produced within the brain itself, and some opioid receptor activity during exercise is real. But when researchers started directly testing whether blocking opioid receptors would prevent runner’s high, the results were surprising: it reduced it somewhat, but didn’t eliminate it. Something else was doing most of the work.

Endocannabinoids: The Thing That Actually Crosses the Barrier

In 2021, neuroscientist David Raichlen at the University of Southern California published a study that did something clever: it compared the neurochemistry of exercise in humans, dogs, and ferrets.

All three species were put through sustained running or exercise. Afterward, researchers measured their levels of endocannabinoids — particularly anandamide — and also checked for signs of reduced anxiety and altered pain perception.

Anandamide is one of the brain’s internally-produced endocannabinoids. It’s sometimes called the “bliss molecule.” It activates the same CB1 receptors that THC (the active compound in cannabis) activates. Crucially, unlike most endorphins, anandamide can cross the blood-brain barrier.

In humans: post-exercise anandamide levels rose significantly, and anxiety dropped. The signs of runner’s high were present.

In dogs: same result.

In ferrets: neither effect occurred.

What Ferrets Tell Us About What You Were Built For

The ferret result isn’t a failure — it’s the key to understanding why exercise feels good at all.

Dogs and humans are both cursorial hunters — animals built for long-distance pursuit. The evolutionary history of both species involves sustained running: chasing prey over extended distances, sometimes for hours. An animal that experiences pleasure and reduced anxiety during sustained running has a competitive advantage. It runs farther, hunts better, and doesn’t give up when the effort becomes painful.

Ferrets are not cursorial hunters. They’re mustelids — ambush predators that hunt in short bursts in enclosed spaces. They don’t run long distances. They never needed a reward system that said yes, keep going, this sustained effort is good.

Their brains don’t have the same endocannabinoid response to sustained running because their evolutionary history never required one.

The runner’s high isn’t a side effect of exercise. It’s a reward system that evolved specifically to motivate the kind of sustained locomotion that our ancestors needed to survive.

BDNF: The Part That Lasts

Beyond the acute high, there’s a longer-term effect that may matter more: exercise dramatically increases production of BDNF — brain-derived neurotrophic factor.

BDNF is sometimes called “fertilizer for the brain.” It promotes the growth and differentiation of neurons, protects existing neurons from damage, and is particularly active in the hippocampus — the brain region central to memory formation and spatial navigation.

Regular exercise reliably increases hippocampal volume. The effect is documented in multiple imaging studies across different populations. In older adults, exercise-induced BDNF production slows age-related hippocampal atrophy. In people with depression, BDNF levels are often low — and antidepressants, among other effects, increase BDNF levels. Exercise does the same.

This is the less flashy but more durable benefit of exercise: not the high, but the structural improvement to the brain that accumulates over months and years.

Why It Works Differently for Everyone

Not everyone gets runner’s high. People vary substantially in their exercise-induced endocannabinoid response, their BDNF sensitivity, and their baseline neurochemistry. Some people find sustained aerobic exercise genuinely euphoric after a threshold is crossed; others find it only uncomfortable, in graded degrees.

Some of this is genetic — there are known variants in the FAAH gene that affect anandamide metabolism, influencing how much anandamide accumulates and how long it stays available after exercise. People with certain FAAH variants are measurably less anxious and more willing to engage in extinction of fear learning — the same people who may find runner’s high more accessible.

Fitness level also matters. The endocannabinoid response to running appears to be more pronounced at moderate intensity rather than maximum effort — which means very unfit people (for whom any running is near-maximum) and very fit people (who need high intensity to reach moderate relative effort) may have different response profiles to the same absolute workout.

The Minimum Effective Dose

For most people, the exercise mood benefit doesn’t require extreme duration or intensity.

Studies consistently find that 20–30 minutes of moderate-intensity aerobic exercise produces measurable improvements in mood, anxiety, and cognitive function. The BDNF benefits accumulate over time — they’re not acute, and require consistent exercise over weeks and months to show structural effects.

The specific “runner’s high” (anandamide-mediated euphoria) requires sustained effort — typically at least 45–60 minutes of aerobic activity — which is why casual walks usually don’t produce it but long runs do.

What’s notable is that the mood benefit appears largely independent of whether you were enjoying the exercise before it started. Studies show people who predict they won’t enjoy a workout frequently report better mood afterward than they expected. The system that benefits from movement doesn’t require that you like the movement first.

It’s a system built for survival, not preference.

You were made for sustained effort. The biology already knows this.

The only requirement is that you start.