Researchers have been able to trigger neural connections in awake mice that normally occur only during sleep, offsetting impacts of sleep deprivation and revealing features key to restorative effects of sleep. "What we're essentially doing is forcing sleep in a local region of the brain. While that part is solidifying memories and restoring learning capacity, other parts stay aware/vigilant and connected to environment," author Chiara Cirelli, a professor of psychiatry at the US' University of Wisconsin-Madison, said.
"Dolphins do something similar, sleeping with only one brain hemisphere at a time," Cirelli said. The researchers' study, published in the journal Nature Neuroscience, used a combination of light-pulsing implants and genetic modifications to induce rhythmic on-and-off activity in one side of the brains of sleep-deprived mice for 30 minutes at a time, mimicking patterns that occur during NREM (non-rapid eye movement) sleep.
NREM sleep forms about 80 per cent of sleep for human adults and is the restful phase of one's sleep cycle when brain activity, heart rate and breathing slows down.
The NREM stage of sleep cycle is also associated with memory consolidation in which new, temporary information is converted into stable, long-term memories.
Junctions between neurons that make memories are evaluated. During the NREM phase, the brain protects important connections for long-term storage, prunes those that are less necessary, and makes space for new ones, the researchers explained.
In previous studies, the team showed that when sleep-deprived, both rats and humans can exhibit a local slow-wave brain activity -- a hallmark of NREM sleep -- while awake.
While the deprivation-induced dips into sleep-like activity may have been too sporadic and brief to be beneficial, the findings raised questions about the possible effects of a longer, more systematic version of this activity, the researchers said. During the study, when the mice subsequently slept, slow-wave activity was lower in the specific brain regions the authors had stimulated, indicating a lesser need for sleep. Further experiments suggested that the effect hinged not on the overall reduction in neuronal firing, which some scientists had suggested was critical to recover from wake-induced neuronal fatigue, but rather on the specific alternating on-and-off pattern of activity. The researchers explored potential benefits through a behavioural test of tactile memory, for which sleep is important.
Sleep-deprived mice which received stimulation in motor and sensory regions on both sides of the brain performed similarly to those who were well rested. Sleep-deprived mice who did not receive stimulation performed significantly worse.
The authors "locally induced alternating on/off periods during wakefulness using optogenetics in mice. This led to a local ipsilateral reduction in SWA (slow wave activity) and synchrony during subsequent sleep, and to reduced markers of synaptic strength." "Moreover, bilateral induction of off periods over sensorimotor cortex during sleep deprivation restored memory consolidation," they said.
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