If you're a new parent or someone who regularly lies awake at night unable to sleep, chances are you've worried about not getting enough rest.
Sleep trackers often add to those concerns. Many devices now display a running estimate of your so-called sleep debt, calculating how much sleep you supposedly "owe" your body.
The problem is that the term itself can be misleading. It suggests that sleep works like a bank account, where missed hours accumulate over time and must eventually be repaid in full.
In reality, sleep does not operate that way. And constantly chasing an ideal number of hours may not actually improve your sleep.
What is sleep debt?
Two systems largely control sleep.
The first is the body's internal clock, which synchronises sleep and wakefulness with the day-night cycle.
The second is known as sleep pressure.
Sleep pressure gradually builds the longer we remain awake and falls while we sleep. When it reaches very high levels, staying awake becomes extremely difficult. Someone who has gone an entire night without sleep, for example, may unintentionally fall asleep.
This is the biological process that the concept of "sleep debt" attempts to describe.
When you sleep less than your body requires, sleep pressure increases. If you are later given the opportunity to sleep longer, you typically will.
In that sense, the debt analogy is useful. However, it does not accurately reflect the complexity of the body's sleep-regulation systems.
Unlike financial debt, which can be measured precisely and remains until fully repaid, sleep pressure is dynamic and constantly adapting.
What happens when we sleep less?
To study sleep deprivation, researchers often keep volunteers in laboratories and restrict their sleep, sometimes to four or six hours per night over several days or even weeks.
These studies show that the body quickly adjusts its priorities.
When sleep becomes limited, the body preserves deep sleep, the stage associated with much of the body's recovery and restoration, while reducing lighter stages of sleep.
People also tend to fall asleep more quickly and spend less time awake during the night.
In other words, when sleep time is restricted, the body becomes more efficient at using the time available.
When participants return to normal sleeping schedules, researchers observe a period of recovery sleep. This generally involves one or two nights of longer and deeper sleep.
After that, the body's immediate need for extra sleep appears largely satisfied.
Importantly, however, people do not simply sleep every hour they previously lost.
In everyday life, this means that after several nights of restricted sleep, most people experience one or two nights of deeper or slightly longer sleep before gradually returning to their normal pattern.
Why do we still feel tired?
Researchers also examine mental functions affected by sleep, including concentration, attention and cognitive performance.
These often recover more slowly than sleep itself.
As a result, a person may have already completed the necessary recovery sleep but still require several normal nights of rest before their mental sharpness fully returns.
This helps explain why people sometimes continue to feel tired despite getting a couple of good nights of sleep.
Can worrying about sleep debt make things worse?
Research suggests that perceptions about sleep can influence how we feel.
One study found that participants who were told their sleep quality had been poor felt more tired and reported worse moods the following day.
Another experiment showed that cognitive performance was influenced by how much sleep people believed they had obtained, regardless of the actual amount of sleep they received.
Although researchers have not directly studied the effects of informing people about their total sleep debt, existing evidence suggests that such information could increase anxiety about sleep.
That anxiety may ultimately make sleeping more difficult.
The problem with calculating sleep debt
There is another challenge with the concept.
To calculate a debt accurately, you first need to know exactly how much sleep you need.
Sleep-tracking devices attempt to estimate this, but sleep requirements vary significantly from person to person.
Some healthy adults function well on around six hours of sleep. Others may need close to nine hours.
These requirements also change over time.
People often need more sleep when they are ill, recovering from physical exertion or undergoing periods of intensive training. Seasonal changes can also influence sleep patterns, with many people sleeping longer during winter.
At the same time, wearable devices estimate rather than directly measure sleep.
Although they are becoming increasingly sophisticated, they still provide approximations rather than perfect assessments.
Essentially, they are comparing one estimate, how much you slept, with another estimate, how much sleep you require.
The bottom line
Sleep debt is a useful way of thinking about how sleep pressure builds when we stay awake for long periods and decreases when we sleep.
However, the body's response to sleep loss is not like a constantly growing bill that must be repaid hour by hour.
The human body is remarkably adaptable. It adjusts, prioritises deep sleep and generally recovers after periods of reduced rest.
Most importantly, accurately calculating a precise sleep debt would require knowing exactly how much sleep you need and exactly how much sleep you obtained, two things that remain difficult to determine with certainty.
The encouraging message is that the body is designed to cope with occasional sleep loss and recover over time.
There is little reason to keep a detailed mental ledger of lost sleep or become obsessed with eliminating every minute of supposed sleep debt.
Source: The Conversation / Reuters
