Scientists at Stanford University say that getting between seven to eight hours sleep each night in old age is the ideal way of keeping your brain healthy. People who regularly got fewer than six hours had worse cognitive function and higher levels of a dangerous plaque in the brain linked to dementia. People who slept too much also performed poorly in memory, reaction time and flexible thinking tests.

Sleep generally happens in four stages. The first three stages are known as ‘non rapid eye movement’ or NREM sleep. The final stage is known as rapid eye movement or REM sleep.

A typical night’s sleep moves backwards and forwards between the stages.

Stage 1: In the first five minutes or so after dropping off we are not deeply asleep. We are still aware of our surroundings but our muscles are starting to relax, our heart beat slows and brainwave patterns, known as theta waves, become irregular but rapid. Although we are asleep during this stage, we may wake up from it feeling like we didn’t sleep at all, but after about five minutes our bodies move into stage two.

Stage 2: This is when we have drifted into sleep, and if awakened would know you had been asleep. This stage is identified by short bursts of electrical activity in the brain known as spindles, and larger waves known as K-complexes, which indicate that the brain is still aware of what is going on around it before turning off to a sub-conscious level. Heartbeat and breathing is slow, and muscles relax even further. Our body temperature drops and eye movements stop. Brain wave activity slows but is marked by brief bursts of electrical activity. At this stage, waking up is still fairly easy.

Stage 3: Stage 3 non-REM sleep is a period of deep sleep that we need to feel refreshed in the morning. It occurs in longer periods during the first half of the night. During Stage 3, our heartbeat and breathing slow to their lowest levels during sleep and brain waves become even slower. Our muscles are relaxed and other people may find it difficult to awaken us. It is during Stage 3 that the body repairs muscles and tissues, stimulates
growth and development, boosts immune function, and builds up energy for the next day.

Hypnagogia — sometimes called Hypnogogic Sleep — is the transition state between wakefulness and sleep and is associated with NREM Stages 

Mental phenomena  during hypnagogia include lucid thought, lucid dreaming, hallucinations and sleep paralysis.

Stage 4: REM sleep: REM sleep first occurs around 90 minutes after falling asleep. Our eyes move rapidly from side to side behind closed eyelids. Mixed frequency brain wave activity becomes closer to that seen in wakefulness and our breathing becomes faster and irregular, and heart rate and blood pressure increase to near waking levels.

Most dreaming occurs during REM sleep, although some dreaming can also occur in nonREM sleep. Arm and leg muscles become temporarily paralysed, which prevents us from acting out our dreams.

As we age, we spend less time in REM sleep. Memory consolidation most likely requires both non-REM and REM sleep.

Your brain pays attention to unfamiliar voices during sleep to stay alert to potential threats — undoubtedly a survival strategy going back tens of thousands of years. Hearing unfamiliar voices wile asleep causes the human brain to become aware during non-rapid eye movement sleep (NREM), the first stage of sleep. Researchers in Austria studied the brain activity of sleeping adults and were able to measure their response to both familiar and unfamiliar voices.

Researchers were not able to see the effect during REM, the deepest stage of sleep, but his was likely due to micro-structure changes in the brain. When we sleep, even though our eyes are closed, the brain continues to monitor the environment, balancing our need to sleep with the need to eventually wake up. One of the ways it accomplishes this is by selectively responding to unfamiliar voices over familiar ones.

The need to quickly awaken goes far back in our evolution, and is a guard against potential danger, characterised by less familiar auditory cues. Unfamiliar voices – like those coming from a TV – prevent a restful night’s sleep because the brain is on higher alert.

Non-REM sleep is when you are sleeping deeply, and when it’s hard to wake up.

Recent research at the University of Salzburg highlighted discrepancies in brain responses to auditory stimuli based on their relevance to the sleeper. The results suggest that unfamiliar voices are a strong promoter of brain responses during NREM sleep.

The study involved 17 volunteers, 14 of whom were female, with an average age of 22. 

None had any sleep disorders. They were fitted with polysomnography equipment which measures brain waves, respiration, muscle tension, movements, heart activity and more, during a full night’s sleep. They were monitored as they advanced through the different sleep stages.

Before the start of the experiment, the participants were advised to maintain a regular sleep/wake cycle — around eight hours of sleep — for at least four days.

As they slept in the university lab, they were presented with auditory stimuli via loudspeakers of their own first name and two unfamiliar first names, spoken by either a familiar voice, (such as a parent) or an unfamiliar voice unknown to them.

The researchers found that unfamiliar voices elicited more K-complexes, a type of brain wave linked to sensory perturbances during sleep, compared to familiar voices. While familiar voices can also trigger K-complexes, only those triggered by unfamiliar voices were found to be accompanied by large-scale changes in brain activity linked to sensory processing.

However, brain responses to the unfamiliar voice occurred less often as the night went on and the voice became more familiar, indicating the brain may still be learning during sleep.

The results suggest K-complexes allow the brain to enter a ‘sentinel processing mode’ where the brain stays asleep but retains the ability to respond to relevant stimuli. It might well be that the sleeping brain learns, through repeated processing, that an initially unfamiliar stimulus poses no immediate threat to the sleeper and consequently decreases its response to it. Conversely, in a safe sleep environment, the brain might be expecting to hear familiar voices and consistently inhibits any response to such stimuli so as not to interrupt sleep.

As well as K-complexes, presenting auditory stimuli during NREM sleep increased the number of ‘spindles’ and ‘micro-arousals’ in the brain — spindles being faster brain waves that appear during NREM sleep and are linked to memory consolidation.

Micro-arousals are periods in sleep during which the EEG signal shifts from slow and synchronised activity of sleep to faster, wake-like activity. By definition, they last from three to 15 seconds — if they are longer they are considered awakenings. They appear in all sleep stages.

Conversely, the researchers found no difference in the amount of triggered K-complexes, spindles or micro-arousals between the subject’s own name and unfamiliar names. This is interesting because previous research, including a study in 1999 by a French team, has demonstrated that the subject’s own name evokes stronger brain responses than other names during sleep.

The research was published in the journal JNeurosci