How long can you stay awake before it becomes harmful?

Everyone has stayed up too late once in a while. Maybe you were finishing work, traveling, or just couldn’t fall asleep. Missing a few hours isn’t the end of the world, but at some point, your body starts showing signs that it’s had enough. So, how long can you stay awake before it becomes harmful?

Most adults need between seven to nine hours of sleep each night for healthy function. Go without sleep for too long, and things begin to unravel physically, mentally, and emotionally.

What happens after 24 hours without sleep

After 24 hours without sleep, your brain starts to slow down. The effects of sleep deprivation can feel similar to having a blood alcohol concentration of 0.10%, which is above the legal limit for driving in many countries. You’ll notice slower reaction times, trouble focusing, and mood swings.

Centers for Disease Control research links this kind of short-term acute sleep deprivation to daytime sleepiness, poor memory, and impaired decision-making. You might also crave high-carb foods, since your body’s trying to stay alert. Even if you think you’re doing fine, your performance drops; you just stop realizing it.

At 36 hours without sleep

A 36-hour period without sleep starts to affect the entire body. Stress hormones rise, blood pressure increases, and coordination weakens. Your immune system begins to struggle, making you more likely to catch viruses.

You might feel bursts of energy followed by extreme fatigue. Some people even experience brief “microsleeps”, seconds-long lapses where your brain shuts down, even if your eyes stay open.

At this point, your mental health symptoms can get worse, too. Anxiety feels sharper, and emotions swing faster. The brain’s logic centers and emotional centers stop working in sync.

At 48 hours without sleep

After 48 hours without sleep, your body starts entering survival mode. You’re likely to experience cognitive impairment, strong confusion, and poor coordination. Your ability to regulate temperature and metabolism decreases, which can negatively affect your digestion and immune system.

Your stress hormones remain high, which can lead to short-term spikes in blood pressure. Some people experience vivid hallucinations or brief dream-like states while awake. You might also start feeling disconnected from reality, as your brain tries to piece together bits of deep sleep while you’re still conscious. This is no longer just exhaustion; it’s severe sleep deprivation.

Beyond 72 hours: the breaking point

Reaching 72 hours without sleep puts your body and mind under serious strain. You might lose track of time, forget basic information, or have trouble forming sentences. Sleep medicine experts say these symptoms resemble those seen in total sleep deprivation experiments.

Longer than that, and the body risks severe consequences. Prolonged sleep deprivation can cause lasting damage to cognitive function and mental health, especially if it becomes chronic.

The risks of chronic sleep deprivation

Not sleeping well for one night is one thing. But chronic sleep deprivation is what really hurts. Sleep-deprived individuals often face increased risk of heart disease, kidney disease, metabolic syndrome, and even colorectal cancer.

Long-term sleep loss affects blood pressure, weight gain, and hormones that regulate appetite and stress. It can also worsen mental health conditions like depression and anxiety. Women tend to feel these effects more strongly, particularly during hormonal shifts.

The Centers for Disease Control and Prevention reports that adults who get less than six hours of sleep regularly are at higher risk for high blood pressure and chronic illness. Even missing small amounts each night adds up to a prolonged sleep debt that your body can’t easily recover from.

Why your body can’t function without enough sleep

During sleep, your body does more than rest. It clears waste from the brain, regulates hormones, and strengthens your immune system. Without enough sleep, your brain can’t process memories properly or manage emotions efficiently.

That’s why sleep deprivation often leads to irritability, forgetfulness, and risky behavior. It also interferes with how your body repairs cells and maintains energy. Skipping sleep doesn’t just make you tired; it also disrupts everything from blood pressure regulation to your immune system response.

Recovery sleep and getting back on track

The good news? Your body can recover from occasional acute sleep loss. A few nights of recovery sleep (longer, deeper rest) can bring energy levels back. But if you’ve gone too long without rest, it may take several days to feel fully normal again.

Stick to a regular sleep schedule and aim for quality sleep in a comfortable sleep environment. Practicing good sleep hygiene, like keeping your room dark, cool, and quiet, helps signal that it’s time to fall asleep.

If you’re struggling to get consistent rest or experiencing sleep disturbances that last for weeks, speak to a healthcare provider. Sometimes, sleep disorders or underlying health conditions play a role.