What is sleep debt and how does it affect your mental and physical health? Do you spring out of bed ready to face the day without the aid of an alarm clock or crying child? Do you have coffee, just because you enjoy the ritual and aroma? If the answer is "no," you're not alone.

Sleep debt is a common ailment many people deal with. The Centers for Disease Control have estimated that one in three people in the U.S is chronically sleep-deprived. During the pandemic, however, people are reporting more time asleep — but the quality of that sleep has been declining.

Even a single suboptimal night’s sleep can lead to serious consequences. It has been estimated that 20 percent of fatal car accidents were directly attributable to the lack of sleep.

An Australian study showed that being awake for 18 hours was cognitively equivalent to having a blood alcohol concentration (BAC) of .05 — and .10 after 24 hours.

Poor sleep for a few nights can cause a lowered immune response and impaired memory and attention can lead to poor judgment, short tempers, and tears. Even our vision is affected.

Prolonged sleep deprivation is not easy to recognize. Sleep deprivation is cumulative. As tiredness becomes normalized, we fail to realize how much we are affected. We risk developing long-term health problems — weight gain, diabetes, hypertension, heartbeat irregularities, stroke, brain fogginess, and depression, to name a few.

Alzheimer’s disease is also associated with sleep deprivation in some studies. Sleep debt cannot be "repaid" with extra sleep. This sleep debt cannot be "repaid" by sleeping in for one weekend, but significant improvement can be made with attention to good sleep hygiene.

What are the stages of healthy sleep?

Sleep is not just shutting one’s eyes and becoming oblivious. The brain repeatedly cycles through two different types of sleep: REM (rapid-eye movement) which most associate with dreaming and the equally important but even less understood, non-REM.

When you first close your eyes and settle down in a relaxed state between being awake and falling asleep, the point where you're most often asked, "Are you asleep?" and you find yourself wondering for a moment, you're in the first stage.

Of course, if you’ve been asked that helpful question, you’ll have to start over. Next is light sleep, when the heart rate drops, breathing slows and deepens, and body temperature lowers. The third and fourth stages are deep sleep.

Non-REM sleep may be more important for sleep debt

Though REM sleep was previously believed to be the most important sleep phase for learning and memory, more recent studies suggest that non-REM sleep is possibly even more important for these tasks.

In fact, early sleep cycles show longer non-REM phases. Moving into REM sleep, we start to dream, the eyes move rapidly behind closed lids, brainwave patterns resemble those while awake and breathing, and heart rate varies more.

Thankfully, voluntary muscles lose tone during this phase, so even while running a marathon, paragliding down a mountainside, or chasing a pot of gold, muscles can rest and repair, too.

The cycle then repeats itself, but with each cycle, less time is spent in the deeper stages and more in REM sleep. Each cycle lasts 90 to 120 minutes with most people needing four to six full cycles, meaning most people need between six and 10 hours of sleep. Everyone's sleep needs are different and dictated by our genes.

Ruth Bader Ginsburg famously slept little, often getting by with less than three hours a night. But while she is a role model for many of us for many things, she was an outlier, as was Einstein who needed 12. Studies indicate that sleep is yet another thing bound to our genes, and while it is possible to "train to have less sleep," it's not possible to "train to need less sleep."

Improve your sleep habits and cancel out sleep debt.

If you suspect that you're one of the one in three people who is sleep-deprived, why not resolve to improve your sleep habits?

• Get some exposure to natural daylight early in the morning
• Get some gentle exercise during the day.
• Don't do anything strenuous in the two hours before sleep.
• Turn off all screens an hour before bed.
• Go to bed at the same time each night — workdays and days off, too.
• Allow yourself to wake up, naturally.
• Set your bedtime to allow for 9 hours of sleep while you catch-up and adjust as needed.

It's important that the cycle be completed — waking up mid-dream can be disconcerting and, over time, bad for you.

Sleep well!