During deep sleep, the brain may be removing the detritus that accumulates during a hard day of thinking, US researchers say.

Researchers have found that during slow-wave sleep, cerebrospinal fluid (CSF) pulses into the brain, taking out the garbage as it recedes, according to a report published in Science.

Using high-speed brain imaging, the researchers mapped the series of events that occur as the brain enters deep sleep and brain waves start to slow and synchronise.

They found that the blood flow to the brain diminishes, allowing for an influx of CSF, washing away the day’s detritus of proteins and other waste substances that might harm the brain if they are not cleared out.