How to learn in your sleep

Sleep has always baffled researchers, but a hypothesis that has emerged in recent years is that one function of those “little slices of death” is the consolidation of memories. A new study by German researchers, published in advance on the Nature website, provides some evidence for this theory, by showing that  electrical stimulation of the brain during sleep can improve performance on a word recall task.

Sleep is a self-induced, reversible state characterized by reduced motor activity, lowered response to sensory stimuli and the adoption of characteristic postures. Until the 1950s, it was thought of as a passive activity, but it is now known that, while the body rests, the brain is highly active  – perhaps more so than when we are awake. 

Sleep involves 5 stages, which we cycle through as the night progresses. A complete cycle lasts 90-100 minutes, so that, during an average sleep, a person will go through 4 or 5 cycles. Each stage of sleep is characterized by a particular pattern of brain waves (left). Stage 1 is a transition between wakefulness and sleep, and is characterized by mixed frequency waves (alpha and theta waves). During this stage, we drift in and out of sleep and can be woken  easily. The eyes move slowly, and many people experience sudden muscular contractions (hypnic myoclonia) preceded by a falling sensation. During stage 2, eye movements stop, and brain activity slows. This stage is characterized by theta waves, with a frequency of 12-14 Hz (cycles per second), interspersed by bursts of increased activity called ‘spindles’. Stages 3 and 4 together are known as ‘deep sleep’, and are characterized by very slow delta waves, of frequency 1-4 Hz. During these stages, there is no eye movement or muscular activity. The final stage is rapid eye movement (REM) sleep. During this stage, breathing becomes rapid and shallow, and the eyes jerk back and forth quickly. REM sleep, during which dreaming occurs, is characterized by ‘theta’ waves, which have a frequency of 4-8 Hz. The first period of REM sleep usually begins about 70-90 minutes after falling asleep; as the night progresses, each successive REM stage increases in length, and the duration of the other stages decreases.

The new study was led by Jan Born, a neuroendocrinologist at the University of Lübeck. It involved 13 medical students, who were asked to memorize 46 pairs of words, as well as a finger-tapping task, before going to sleep. A cap containing four electrodes was placed over the participants’ heads, and an electroencephalogram was used to monitor their brain waves. In various trials conducted over several nights, electrical stimulation at a frequency of 0.75 Hz was applied during different stages of sleep; the  stimulation was applied in 5 minute periods, with pauses of 1 minute in between, and induced in the student the slow wave oscillations characteristic of sleep stages 3 and 4.

Upon awakening, the students were asked to recall as many of the word pairs as they could. It was found that stimulation applied 4 minutes before the students entered stage 2 of sleep was most effective. The effect of electrical stimulation was small, but significant – on average, they remembered 4.8 more words (an increase of about 8%)  than controls whose brains had not been stimulated.   

The electrical stimulation did not, however, aid in the recall of the finger-tapping task. Thus, Born and his colleagues concluded that inducing slow wave oscillations could help to consolidate declarative (or factual) memories, but not memories of motor skills (of which the finger-tapping task was an example).

Previous work has shown that the brain produces more slow wave oscillations when one has learnt a new task before going to sleep. This study adds weight to the link between these brain waves and memory consolidation. Born hypothesizes that inducing slow wave oscillations by triggering within neurons the signals required for strengthening synaptic connections. Some researchers, however, are skeptical of any causal link between sleep and memory.

Electrical stimulation of the brain may one day be used as a therapy for condtions in which memory loss is a symptom. But, because any adverse side effects are as yet unknown, an apparatus that could be used to enhance memory in healthy people (for example, medical students doing some last-minute revision) will, at least for now, remain a dream.