Tossing, turning, forgetting

Continuity of sleep, not just the total hours of nightly slumber, is crucial to forming and retaining memories, a new study in mice suggests.

Mice couldn’t remember objects they’d seen before after a night of interrupted sleep, Asya Rolls of Stanford and her colleagues report online July 25 in the Proceedings of the National Academy of Sciences. Even though the mice got just as much sleep as normal and slept as intensely as usual, breaking that sleep into one-minute chunks was enough to erase the memory of toys the animal had seen before.

The results emphasize that sleep is a process, says Paul Shaw, a neuroscientist at Washington University in St. Louis who was not involved in the study. “Whatever biological function sleep serves takes time,” he says. “So if you wake up, you disrupt that process and have to start from scratch again.”

Scientists already had inklings that continuous bouts of sleep were important for learning and memory, Shaw says. But previous experiments had disrupted sleep in ways that made it hard to tell whether learning and memory problems stemmed from fragmented snoozing or from stress or other confounding conditions. In the new study, the Stanford researchers used a “really cool” genetic trick to interrupt the mice’s sleep without all the problems associated with previous studies, Shaw says.

Rolls and her colleagues introduced a light-sensitive protein called channelrhodopsin-2 into certain brain cells. Shining a pulse of blue light on the cells through fiber-optic cables implanted in the brain activated the cells and briefly woke the animals. Outwardly, the mice didn’t even appear to wake up. “They maybe just twitched a muscle,” says Rolls. But the researchers could detect the brief arousals by monitoring the mice’s brain waves.

Mice that got continuous sleep or that were woken up every two minutes remembered objects they’d seen before. The mice crawled on, sniffed, tasted and played with new objects far more than familiar objects. Mice that woke up every minute explored old objects just as much as new toys, indicating that the animals didn’t remember which objects they had encountered before.  

Humans and other animals may need much more than two minutes of uninterrupted sleep to keep memories intact, Shaw says. Mice typically sleep only a few minutes at time. Fruit flies in Shaw’s lab seem to need at least 30 minutes of continuous sleep to learn and remember things. People may need even longer snooze periods.

“There is some sort of ongoing process that if you interrupt, you can’t consolidate the memory,” Rolls says. “We don’t know what that is.”

It’s not clear what happens between one and two minutes of sleep that helps mice remember, Rolls says. One idea is that waking up prematurely may disrupt the replay of the day’s events that happens during sleep to strengthen memories. Breaking sleep into tiny bouts may mean that the memory is never fully replayed, so it doesn’t get strengthened, or even gets erased, Rolls says. Interrupting sleep might also interfere with protein production and the rewiring of brain cells, both of which are thought to happen during sleep, she says.