The answer depends on whether you’re talking about your personal perception of your ability to function or your actual ability to function. “Research has shown that at the beginning of some sleep deprivation [meaning anything less than the recommended seven to nine hours of sleep], people frequently report that they have issues with cognitive and emotional functioning, like not being able to think as fast, struggling with memory, or being more irritable,” says clinical psychologist Shelby Harris, PsyD, author of The Women’s Guide To Overcoming Insomnia. “But after prolonged sleep deprivation, they report that, in their own perception, these issues even out, and they can perform just fine.”

Unfortunately, research shows that perception doesn't mesh with reality. A 2003 study designed to determine whether hours of sleep per night could be reduced gradually over time without consequences found that people who slept for six hours or fewer per night for two weeks experienced the same level of cognitive decline as those who didn’t sleep *at all* for two nights in a row—but their subjective ratings of how sleepy they felt didn’t reflect that decline.

As a result, psychologists suspect that there’s a big gap between how people think sleep loss is affecting them over time (not much) and how it’s actually affecting them (a lot). “When people are chronically sleep-deprived, they may just stop noticing the effects of the lack of sleep,” says behavioral sleep medicine psychologist Jade Wu, PhD, sleep advisor at Mattress Firm. They could then become over-confident in how well they’re really functioning—much like the drunk person who thinks they’re fine to drive home when they definitely are not, says Dr. Harris.

That’s not to say that you won’t be able to function well after a single sleepless night or two. “If there is a short period of time when sleeping less is required, you will be able to temporarily reduce sleep and then make up for the lost sleep later, such as during the weekend,” says neurologist and sleep specialist Temitayo Oyegbile-Chidi, MD, PhD, chair of the National Sleep Foundation. But over time, the cognitive deficits triggered by sleep loss will always grow in an accumulative fashion if you continue to restrict your sleep, regardless of whether you’re aware of that, she adds.

There’s no evidence that the body can adapt to less sleep, either. “With chronic sleep loss, metabolism becomes dysfunctional, too, which increases the chance of stroke, heart disorders, and diabetes,” says Dr. Oyegbile-Chidi.

In general, different stages of sleep serve key restorative purposes across the body, doing things like revving up your immune system and decreasing pressure on the heart. If you don't get the recommended seven to nine hours of sleep, particularly for an extended period of time, you’re cutting short these processes, leaving the body less and less able to recover from the daily wear and tear of life.

Why it might *feel* like you can adapt to less sleep over time

We've established that your perception of how well you function is distorted when you are sleep deprived. But the body will also try to compensate for sleep deprivation by going into physiological overdrive, says Dr. Wu. “Sleep deprivation is a stressful state, and this stress tells the body that we’re in survival mode,” she says. “To help us survive this crisis, the body revs up with adrenaline and cortisol, so we can be ready to fight or flee.”

This hormonal surge could leave you feeling hyped and fully competent. (If you’ve ever been totally alert after an all-nighter, you know the feeling.) But that additional alertness isn’t sustainable long-term. “If we’re constantly in this survival mode, the hyper-arousal becomes chronic and will start to wear the body down,” says Dr. Wu.

Consuming any stimulant drug (like good old caffeine) can also mask feelings of sleepiness and its negative cognitive effects, too, adds Dr. Wu, leaving you feeling alert and focused despite sleep deprivation. But again, this is still just a temporary fix.

While there is some variation in different peoples’ sleep needs based on genetics, and the need for sleep does lessen slightly with age, the general recommendation for adults is seven to nine hours for a reason: The vast majority of people function optimally when their nightly sleep falls in this range.

The truth is, there’s no real way to cheat your need for sleep, given the very real physical and cognitive purposes that sleep serves. “For years, the general culture and belief was that sleep was basically lost time—a ‘shutdown’ process, rather than a ‘reset’,” says Dr. Oyegbile-Chidi. But as the science on sleep has advanced over the past few years, so has the public’s understanding of its importance. “Slowly but surely, people are beginning to understand how essential good sleep is for good health,” she says, “and how it needs to be prioritized.”