If sleep were a credit card company, many of us would be in deep trouble.

Medical evidence suggests that for optimum health and function, the average adult should get seven to nine hours of sleep daily. But more than 60% of women regularly fall short of that goal. Although each hour of lost slumber goes into the health debit column, we don’t get any monthly reminders that we’ve fallen in arrears.

In fact, the greater the sleep debt, the less capable we are of recognizing it: Once sleep deprivation — with its fuzzy-headedness, irritability, and fatigue — has us in its sway, we can hardly recall what it’s like to be fully rested. And as the sleep debt mounts, the health consequences increase, putting us at growing risk for weight gain, diabetes, heart disease, stroke, and memory loss.

In some cases, sleep debt results from insomnia or other underlying conditions that may require medical attention. But most sleep debt is due to burning the candle at both ends — consistently failing to get to bed on time and stay there until we’ve slept enough.

Fortunately, sleep doesn’t charge interest on the unpaid balance, or even demand a one-for-one repayment. It may take some work, but you can repay even a chronic, longstanding sleep debt.

How we sleep

We need sleep, and, in a sense, we’re programmed to be sure that we get it. The body summons sleep in two ways: by boosting circulating levels of the neurotransmitter adenosine and by sending signals from the circadian clock, which controls the body’s daily rhythms. Together, these two systems establish an ideal bedtime for each of us.

Adenosine is partly a by-product of the cells’ energy expenditure. As our cells produce power to move us through the day, adenosine is released into the bloodstream and taken up by receptors in the brain region that governs wakefulness (the basal forebrain). There, it acts like a dimmer switch, turning down many of the processes associated with wakefulness, such as attention, memory, and reactions to physical stimuli. As brain levels of adenosine mount, we feel drowsier. (Caffeine keeps us awake by blocking adenosine receptors in the brain.) When we sleep, our energy needs fall, and the level of circulating adenosine drops. After a good night’s sleep, the level is at its lowest, and we are most alert.

The circadian clock regulates all body functions — not just the pattern of sleeping and waking during the 24-hour cycle, but also fluctuations in body temperature, blood pressure, and levels of digestive enzymes and various hormones. Most of us experience a major “sleepiness” peak between 12 a.m. and 6 a.m. and a minor one between 2 p.m. and 4 p.m. Of course, individuals vary. The larks among us might be ready for bed at 9 or 10 p.m. and awake at 5 a.m., while some night owls don’t fall asleep until well after midnight and prefer sleeping until noon.

Why we need sleep

Although sleep doesn’t trump food and water in the hierarchy of physical needs, we can’t live without it. Given the ethical limits on research involving human subjects, scientists have no direct evidence on how extended sleeplessness — that is, beyond a few days — affects human beings. Laboratory rats, however, have been deprived of sleep for long periods, and after a week or two, the results include loss of immune function and death from infections.

In a landmark study of human sleep deprivation, University of Chicago researchers followed a group of student volunteers who slept only four hours nightly for six consecutive days. The volunteers developed higher blood pressure and higher levels of the stress hormone cortisol, and they produced only half the usual number of antibodies to a flu vaccine. The sleep-deprived students also showed signs of insulin resistance — a condition that is the precursor of type 2 diabetes and metabolic slowdown. All the changes were reversed when the students made up the hours of sleep they had lost. The Chicago research helps to explain why chronic sleep debt raises the risk of obesity, heart disease, stroke, and diabetes.

Sleep loss exacts a toll on the mind as well as the body, as shown by a study done at the University of Pennsylvania and Harvard Medical School. The researchers studied 48 healthy men and women, ages 21 to 38, who had been averaging seven to eight hours of sleep nightly. They assigned three-quarters of the volunteers at random to three different groups that slept either eight, six, or four hours nightly; a fourth group agreed to go without sleep for three days. Every two hours during their waking periods, all the participants completed sleepiness evaluation questionnaires and took tests of reaction time, memory, and cognitive ability.

Over the course of two weeks, reaction times in the group that slept eight hours a night remained about the same, and their scores on the memory and cognitive tasks rose steadily. In contrast, scores for the four-hour and six-hour sleepers drew closer to those of the fourth group, whose scores had plummeted during their three days without sleep. After two weeks, the four-hour sleepers were cognitively in no better shape than the sleepless group after its first night awake. Their memory scores and reaction times were about on par with those of the sleepless after their second consecutive all-nighter. The six-hour sleepers performed adequately on the cognitive test but lost ground on reaction time and memory, logging scores that approximated those of the sleepless after their first night awake.

Meanwhile, the six-hour and the four-hour sleepers were failing to gauge reliably how sleepy they had become. At the end of the study, their self-rated sleepiness scores were leveling off, even as their performance scores continued to decline.

Getting enough sleep is central to living your best life—from staying safe on the road to being productive on the job. Adults typically need seven to nine hours a night, but for about 40 percent of Americans, that much shuteye is an elusive goal. The average is more like 6.8 hours—12 minutes shy of the minimum recommendation. A difference of just 12 minutes a night may not seem like a lot, but over time those minutes can add up to a huge sleep debt (especially if you really need eight or nine hours).

Over time those deficits can take a toll on your health, too, raising your risk of weight gain, diabetes, heart disease, and memory loss. And when you’re in the grips of sleep debt, you often don’t even remember what it’s like to feel well rested, so you may not realize just how tired you are.

So what do you do if you’ve spent weeks or even years of your life logging fewer hours of sleep than you need? You might think that closing the blinds and turning off your alarm on Saturday morning would do the trick, but sleeping in too much on weekends can actually do more harm than good, throwing off your circadian rhythm and making it harder to fall asleep on Sunday night. And that could mean starting your Monday with an even bigger sleep deficit than before.

Luckily, with some persistence, you can chip away at your sleep debt. These are some simple ways to do it.

Start with bedtime.

Rather than sleeping in later, which can make it harder to fall asleep the next night, go to bed earlier and wake up at your normal time. If it feels like there isn’t enough time in the day to get everything done, tracking exactly how you spend your time each day for a week may help you uncover extra minutes—or even hours.

Go slow and steady.

To avoid tossing and turning, try backing up your bedtime by 15 minutes per night to gradually shift your body’s clock. On the weekends and vacations, avoid sleeping in more than a couple of extra hours or it could interfere with nighttime sleep.

Be careful with naps.

Like sleeping in on the weekends, napping can be a quick way to lop a few hours off your sleep debt. But also like sleeping in, if you nap too long in the afternoon, you might have trouble falling asleep at night. And that can mean two steps forward, three steps back. For the best rejuvenation, go for either a 20-minute power nap or a longer 60- to 90-minute snooze.

Commit to healthier sleep habits.

Once you figure out how much sleep you need to feel well rested during the day, think about what changes you need to make in your life to reach that goal. Would making dinner ahead of time with a slow cooker or ordering healthy takeout more often help you get to bed earlier? Would exercising in the morning instead of after work help you to wind down in the evenings? With a few small tweaks, you’ll be back in business.

It might take time to adjust your lifestyle, but the payoff of attacking each day with the sleep you need is worth it.  If you feel like you cannot get the sleep health that you want and need, it may be time to talk to your doctor about sleep apnea.  Testing and treating sleep apnea not only improves your sleep; but it will improve your LIFE!