Not getting enough sleep has become an epidemic, with many of us sleeping significantly less than the 7h - 9h recommended by the National Sleep Foundation. Unfortunately, in our western society, sleep is often treated as a commodity that can be traded for other activities. Chronic short sleep is linked to lower productivity and higher mortality risk. The Rand Corporation estimates the economic costs of sleep deprivation to be up to 3% of GDP. And studies suggest that chronic short sleepers do not require less sleep than other adults; rather, these individuals gradually accrue sleep debt over time.
Sleep is a vital process that contributes to neural restoration and physiological maintenance across multiple systems. For example, sleep is linked to clearance of metabolic waste from the brain and enhancement of cognitive function. It is well documented that sleep contributes significantly to the process of learning and memory.
SHORT SLEEP REDUCES OUR VIGILANCE
In any goal-directed behaviour, we need to be able to maintain our attention for a period of time. Restricted sleep reduces our alertness and therefore affects our cognitive performance. The Psychomotor Vigilance Task (PVT) is a task that is often used by researchers to measure alertness and our ability to sustain attention is. The PVT is a simple task where participants see a dot on a screen, and press a button when the dot arrives. The arrival of the dot is unpredictable so participants need to be vigilant. The task measures reaction time, or speed, to press the button and lapses - not pressing the button when the dot arrives.
Gregory Belenkey and his colleagues from the Walter Reed Army Institute of Research used the PVT task to measure the effects of short sleep on 66 young to middle-aged volunteers. The volunteers were assigned to seven nights of 3h, 5h, 7h or 9h of time in bed. The 5h and 7h are definitely in the range of “normal” time in bed for many people. Following the 7 nights of short sleep, the volunteers had three recovery nights of 8h time in bed.
At 3h sleep per night, the speed on the PVT task declined steadily and lapses or errors of omission increased steadily and significantly over the 7 days. And while speed and lapses improved quickly following the first recovery night, they never fully recovered but remained below baseline for the remainder of the nights.
Regularly getting only 6h sleep a night can compromise our alertness that cannot be made up on a weekend of "normal" sleep. Not feeling sleepy doesn't mean that our attention is not compromised: We underestimate the impact of our sleep deprivation and overestimate our levels of alertness and ability to perform various cognitive tasks.
At 5h and 7h time in bed per night, speed decreased initially and then stabilized at the lower level. Lapses increased only in the 5h group. During the recovery period, the speed and lapses for the 5h and 7h group showed no recovery: they remained at the lower, stable level. Note that 7h time in bed likely translates into 6h of sleep.
In addition, this study found that people are not good at accurately judging how affected they are by chronic sleep restriction. Only the 3h group rated themselves as sleepy, yet the 5h and 7h groups were also cognitively affected!
This study shows that after seven nights of restricted sleep, vigilance does not return to baseline even following 3 nights of “normal” sleep. Other studies have found that PVT performance remains substandard even after 5 or 7 recovery nights of 8 h time in bed. So our belief that we can make up the sleep we lost during the week on the weekend is simply not true.
In addition, this study suggests that people who are chronically sleep deprived underestimate the impact of sleep restriction and may overestimate their levels of alertness and ability to perform various cognitive tasks.
OTHER EFFECTS OF CHRONIC SHORT SLEEP
Micro sleeps: Restricted sleep could lead to micro sleeps - short periods of half a second to several seconds of no response - that people may not even realize they are having. This would be particularly devastating when driving.
Brain injury: There is some evidence that chronic short sleep may have long-term consequences on brain structure. One study found that after 4 weeks of short sleep, there was a 40% loss in locus coeruleus neurons which did not recover even after 1 month of “normal” sleep. The locus coeruleus neurons are a collection of brainstem neurons essential for vigilance.
Another study found that 4 to 7 nights of continuous short sleep suppressed neurogenesis - or the growth of new neurons - in the hippocampus. The hippocampus is one of the most plastic regions of the brain and hippocampal neurogenesis is critical for optimal learning and memory.
A nap can improve alertness and cognitive performance.
Brooks and Lack (2006) examined a variety of nap times following 5 nights of restricted sleep (5 h time in bed). They found that the ideal nap time was 10 min. Compared to pre-nap baseline, there was an increase in a number of cognitive measures following the 10 min nap, and this improvement was maintained for 155 minutes for a variety of measures. Longer naps showed some sleep inertia, meaning that performance decreased upon waking, but then improved.
SUMMARY AND CONCLUSION
Not getting enough sleep, either purposely or due to circumstances, has become a habit for many of us. And we’ve seen how damaging it can be. Chronic short sleep reduces our alertness and ability to sustain attention that affects our cognitive performance. When we’re tired from lack of sleep, despite our best efforts to stay awake, we can experience micro sleeps that can be particularly devastating when driving, for example. There is also some evidence that habitually not getting enough sleep can lead to structural brain changes that are not always reversed, even after one month of normal sleep.
We’re also extraordinarily poor at judging how sleep deprived we really are. Because we don’t feel sleepy, we think we’re alert and able to perform cognitive tasks normally when, in fact, they’re impaired. And finally, many of us try to make up for a week of not getting enough sleep by sleeping more on the weekends. But, a weekend of “normal” sleep does not make up for a week of sleep deprivation.