10. Text 1: Sleep deprivation
Millions of people who suffer sleep problems also suffer myriad health burdens. In addition to emotional distress and cognitive impairments, these can include high blood pressure, obesity, and metabolic syndrome. ‘In the studies we’ve done, almost every variable we measured was affected. There’s not a system in the body that’s not affected by sleep,’ says University of Chicago sleep researcher Eve Van Cauter. ‘Every time we sleep-deprive ourselves, things go wrong. '
A common refrain among sleep scientists about two decades ago was that sleep was performed by the brain in the interest of the brain. That wasn’t a fully elaborated theory, but it wasn’t wrong. Numerous recent studies have hinted at the purpose of sleep by confirming that neurological function and cognition are messed up during sleep loss, with the patient’s reaction time, mood, and judgement all suffering if they are kept awake too long.
In 1997, Bob McCarley and colleagues at Harvard Medical School found that when they kept cats awake by playing with them, a compound known as adenosine increased in the basal forebrain as the sleepy felines stayed up longer, and slowly returned to normal levels when they were later allowed to sleep. McCarley’s team also found that administering adenosine to the basal forebrain acted as a sedative, putting animals to sleep. It should come as no surprise then that caffeine, which blocks adenosine’s receptor, keeps us awake. Teaming up with Basheer and others, McCarley later discovered that, as adenosine levels rise during sleep deprivation, so do concentrations of adenosine receptors, magnifying the molecule’s sleep-inducing effect. ‘The brain has cleverly designed a two-stage defence against the consequences of sleep loss,’ McCarley says. Adenosine may underlie some of the cognitive deficits that result from sleep loss. McCarley and colleagues found that infusing adenosine into rats’ basal forebrain impaired their performance on an attention test, similar to that seen in sleep-deprived humans. But adenosine levels are by no means the be-all and end-all of sleep deprivation’s effects on the brain or the body.
Over a century of sleep research has revealed numerous undesirable outcomes from staying awake too long. In 1999, Van Cauter and colleagues had eleven men sleep in the university lab. For three nights, they spent eight hours in bed, then for six nights they were allowed only four hours (accruing what Van Cauter calls a sleep debt), and then for six nights they could sleep for up to twelve hours (sleep recovery). During sleep debt and recovery, researchers gave the participants a glucose tolerance test and found striking differences. While sleep deprived, the men’s glucose metabolism resembled a pre-diabetic state. ‘We knew it would be affected,’ says Van Cauter. ‘The big surprise was the effect being much greater than we thought.’
Subsequent studies also found insulin resistance increased during bouts of sleep restriction, and in 2012, Van Cauter’s team observed impairments in insulin signalling in subjects’ fat cells. Another recent study showed that sleep-restricted people will add 300 calories to their daily diet. Echoing Van Cauter’s results, Basheer has found evidence that enforced lack of sleep sends the brain into a catabolic, or energy-consuming, state. This is because it degrades the energy molecule adenosine triphosphate (ATP) to produce adenosine monophosphate and this results in the activation of AMP kinase, an enzyme that boosts fatty acid synthesis and glucose utilization. ‘The system sends a message that there’s a need for more energy,’ Basheer says. Whether this is indeed the mechanism underlying late-night binge-eating is still speculative.
Within the brain, scientists have glimpsed signs of physical damage from sleep loss, and the time-line for recovery, if any occurs, is unknown. Chiara Cirelli’s team at the Madison School of Medicine in the USA found structural changes in the cortical neurons of mice when the animals are kept awake for long periods. Specifically, Cirelli and colleagues saw signs of mitochondrial activation – which makes sense, as ‘neurons need more energy to stay awake,’ she says – as well as unexpected changes, such as undigested cellular debris, signs of cellular aging that are unusual in the neurons of young, healthy mice. ‘The number [of debris granules] was small, but it’s worrisome because it’s only four to five days’ of sleep deprivation,’ says Cirelli. After thirty-six hours of sleep recovery, a period
during which she expected normalcy to resume, those changes remained.
Further insights could be drawn from the study of shift workers and insomniacs, who serve as natural experiments on how the human body reacts to losing out on such a basic life need for chronic periods. But with so much of our physiology affected, an effective therapy − other than sleep itself – is hard to imagine. ‘People like to define a clear pathway of action for health conditions,’ says Van Cauter. ‘With sleep deprivation, everything you measure is affected and interacts synergistically to produce the effect.’
Q: In the third paragraph, what idea is emphasised by the phrase ‘by no means the be-all and end-all’?
A. Sleep deprivation has consequences beyond its impact on adenosine levels.
B. Adenosine levels are a significant factor in situations other than sleep deprivation.
C. The role of adenosine as a response to sleep deprivation is not yet fully understood.
D. The importance of the link between sleep deprivation and adenosine should not be underestimated.