It may one day be possible to reap some of the benefits of sleep without ever closing our eyes. Stimulating specific brain activity in awake mice led to some of the same effects as deep sleep, including a boost in memory.

“It should be possible, at least in theory and to some extent, to replicate these results in our species,” says at the University of Oxford, who wasn’t involved in the research. “It would be fascinating to explore whether artificially inducing [this activity] during waking [hours] in humans can result in a subjective feeling of being more refreshed and rested afterwards.”

Sleep is thought to be an essential way for the brain to carry out most of its maintenance work. This includes synaptic homeostasis, the process whereby the brain declutters the thousands of new neural connections made during the day – storing important ones and weakening or cutting away ones that aren’t as necessary.

During non-rapid eye movement (NREM) sleep – the deep sleep state that makes up around 80 per cent of sleep in adults – the brain’s cortex repeatedly fires signals at the exact same time and then shuts those neurons off, in a pattern called slow-wave sleep activity. “This has been linked to synaptic homeostasis, and may be a key mechanism underlying sleep’s restorative functions,” says at the University of Wisconsin-Madison.

Cirelli and her colleagues wondered if a small part of the cortex could be nudged into entering this deep sleep state while an individual is still awake. Some animals do this naturally, such as dolphins, ducks and fur seals, in which one half of the brain enters NREM sleep while the other remains alert and vigilant for predators.

To see if a similar state could be induced, the researchers genetically engineered mice so their neuronal activity could be switched off using light. They implanted a probe into one half of their brain and kept the mice awake for five hours by giving them new things to explore. Near the end of this time period, the light probe was repeatedly turned on and off for 30 minutes, mimicking NREM sleep.

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Afterwards, when the mice were allowed to sleep, brain recordings showed that the stimulated side of the brain didn’t show the usual signs of exhaustion caused by sleep deprivation. “Because that small part of the brain did its decluttering while awake, it no longer needed extra deep sleep afterwards,” says Cirelli.

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Next, the researchers wondered whether forcing sleep during wakefulness boosts memory. So they placed the genetically modified mice in a square box with carpet that had the same texture on both sides of the container. After 15 minutes of exploring the space, the mice were assigned to either a sleep group, a group that was sleep-deprived for 1 hour, or a group that was sleep-deprived for 1 hour but received the artificial deep-sleep stimulation.

The next day, the mice went back into the box, but one side of the container had a new texture. Mice are naturally drawn to novelty, so the researchers measured how much they remembered the old environment by the amount of time they spent on the new side. They found that the sleep-deprived mice that received no stimulation seemed to struggle to tell the new and old side apart, while both the sleep group and the sleep-deprived mice that received the stimulation spent more time on the new side.

The team plans to study whether similar effects could come about in people if this brain activity were induced non-invasively via transcranial electrical stimulation. However, Vyazovskiy stresses that sleep can probably never be replaced. “Sleep is of two kinds – NREM and REM [rapid eye movement] – and we still do not know what it is about the alternation of these two states that makes sleep complete,” he says.

Different people may experience one of five types of sleep, and these profiles each highlight how our shut-eye affects our health.

Previous research has found associations between sleep and ,  and physical conditions, such as . But these studies often looked at the relationship with just one aspect of sleep, such as its duration or quality.

To take a more holistic approach, at Concordia University in Montreal, Canada, and her colleagues analysed the association between seven factors related to sleep – such as sleep satisfaction and the use of sleeping aids – and 118 other measures, including cognition, substance use and mental health. They collected data including cognitive tests, sleep surveys and brain scans from 770 healthy adults aged between 22 and 36 in the US.

From this, the researchers identified five distinct sleep profiles. The first was characterised by a general pattern of poor sleep – greater sleep disturbances, decreased sleep satisfaction and taking a longer time to fall asleep – and worse mental health, such as depression and anxiety symptoms, as well as anger, fear and stress.

The brain scans of people in this group also showed decreased connectivity between networks involved in self-reflection, such as the temporoparietal network, and those responsible for attention and tasks, such as the somatomotor and dorsal attention networks. This could indicate disruptions in the brain’s ability to shift between the inner and outer world, according to the researchers. For instance, people in this category may ruminate on their thoughts and feelings, rather than focusing on their external environment.

People in the second sleep profile also showed signs of worse mental health, particularly related to inattention, but not sleep difficulties. Rather, they had decent sleep overall. “We have interpreted this as sleep resilience,” says Kebets. “So worse mental health, which doesn’t necessarily affect sleep.” People who fit this profile also lacked the brain connectivity patterns seen with the first group, which suggests these patterns are specifically related to sleep issues, rather than overall mental health.

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Meanwhile, the third profile exhibited a relationship between using sleep aids – such as prescription medication or even just teas marketed as helping sleep – and poorer memory and emotional recognition, which is the ability to identify someone’s emotional state through cues such as their expressions or body language. This could explain why people who fit this profile had decreased connectivity in brain regions involved with vision, memory and emotion.

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The fourth profile was distinguished by getting fewer than 7 hours of sleep a night, . This was associated with worse accuracy and longer reaction times on cognitive tests measuring emotional processing, language and social skills. This profile was also associated with more aggressive behaviours and increased connectivity across brain networks. Past studies of sleep deprivation have found , suggesting this is indicative of sleep debt.

Aggression also occurred in the fifth profile, which was characterised by sleep disturbances, such as waking multiple times a night. These disturbances were associated with worse language processing and working memory, as well as signs of poorer mental health, including anxiety symptoms and substance misuse.

These findings bring us closer towards understanding the complex relationship between sleep and health, says Kebets. “Sleep is so central to your sense of well-being, and it’s related to cognition, to physical health, to mental health, to substance use – so many aspects of your functioning.”

But not all of the participants fit neatly into one profile, says Kebets. The researchers also didn’t find evidence of causal relationships between sleep profiles and traits, only of associations. There will no doubt also be a significant proportion of people who regularly experience good-quality sleep. In addition, the participants were mostly white, so the study may have missed sleep profiles that exist among other ethnicities.

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