Sleep debt, simply defined, is the amount of sleep that you owe your body over the past 14 days or so. It’s a running total of the hours of sleep you’ve missed, relative to your sleep need. To better explain how it appears and builds, it’s helpful to look at a debt-free cycle of sleeping and waking.
If you were getting the perfect amount of sleep every night, you wouldn’t have any excess sleep debt. But, you’d still be experiencing a benign buildup of sleep pressure throughout your day, until its peak at bedtime. The sleep-wake cycle relies on sleep pressure to operate normally—if you never got tired at night, you wouldn’t sleep!
If you’re wondering why you get tired at all, you can blame adenosine. Scientists have correlated sleep pressure with this organic compound, which accumulates in your brain every minute you’re awake. As its concentration rises, it decreases your arousal, leading to drowsiness. But when you sleep, the brain purges itself of adenosine, effectively resetting the pressure balance at 0 for the next day.
In the two-process model of sleep regulation, this daily buildup and nighttime depletion of sleep pressure is a process called sleep homeostasis. A homeostatic process is all about balance—a return to a desired stasis. The model says that this regulatory system exists in each of us, and pushes us toward the ideal amount and depth of sleep.
Think of the sleep homeostat like a seesaw in your brain that wants to be level. As sleep pressure and adenosine build on one end and the balance becomes upset, eventually the seesaw tries to even itself out. This homeostatic process has been observed in several animals other than humans. Rats and mice, for example, both tend to sleep for a notably extended period of time following a bout of sleep deprivation.
Whether you’re a person or a rodent, if you’re sleep-deprived, you aren’t getting that full adenosine purge. Instead, you carry over the leftover adenosine to the next day, and the next, compounding the negative effects of sleep loss until you can make an effort to rebalance the seesaw. When you’re saddled with this adenosine and all of its consequences, you’re accruing sleep debt, rather than responding to normal and healthy sleep pressure.
The repercussions of sleep debt are incredibly alarming. For example, after 10 consecutive nights of getting seven hours of sleep—one hour less than the recommended eight—your brain becomes as impaired as it would if you hadn’t slept for a full 24 hours.
If that doesn’t sound too bad, consider that 18 hours without sleep results in the same cognitive impairment as having a blood alcohol level of 0.05%. A full 24 hours without sleep puts you in the position of someone with a BAC of 0.10%—higher than the legal limit in every state.
Furthermore, if you’re someone who drives often, your odds of crashing your car shoot up alongside your sleep debt. The AAA Foundation for Traffic Safety has found that up to ⅕ of all fatal crashes involve a sleep-deprived driver.
Drivers who have slept for less than seven hours the night before dramatically increase their odds of crashing. Compared to those who slept at least seven hours, those who slept for 5–6 hours were about twice as likely to crash, whereas those who slept for four hours or less were 11 times more likely!
Thankfully, sleep debt is actionable—you can make it up, and doing so reverses the impairments caused by sleep deprivation. In what’s considered the canonical study on sleep debt, participants who had only been sleeping 4.5 hours a night for a full week experienced a dramatic boost in cognitive performance and mood after they were allowed two full nights of recovery sleep.
Keep in mind that sleep debt is a cumulative measure. One night of no sleep shouldn’t upset your entire life, just as one night of good sleep won’t help much if you’re chronically underslept. Monitoring your total sleep debt over time is the only way to get an accurate picture of your sleep need.