Why would evolution decree that we should sleep? To lose consciousness and even lose control of our muscles. This would make us vulnerable and endanger our survival. Clearly there must be benefits that are important. Sleep has been shown to help our immune system and muscle recovery. It allows us to process emotions and reduce stress. It stimulates cell growth. It can make us more productive and increase exercise performance.

All of these might be achieved while we are awake but resting. We need to sleep and lose consciousness to consolidate our memories of the day. Not all sleep helps.

The Pattern of Sleep

Each night we will go through many sleep cycles. Cycles last about ninety minutes so we may have up to six cycles. Each will have the same stages but as the night goes on the mix changes.

In the first stage we move from wakefulness to sleep. Our heart rate, breath and eye movements all slow. This stage takes only one to five minutes. In the second stage we drop into deep sleep. Our muscles relax further, and our eye movements stop. Our body temperature falls. During the first sleep cycle of the night this stage can last for twenty-five minutes. Each successive cycle extends this period, and we can spend up to half of our night’s sleep in this state.

In the third stage there is a fundamental change in the pattern of our brain waves. In “slow wave activity (SWA)” our heartrate, breath and brain all slow. Our brain pattern simplifies to a slow wave. This is the sleep from which we find it most difficult to wake. If we do wake, we may feel mentally groggy for up to half an hour. This kind of sleep is concentrated in the first half of the night. In later cycles SWA is much shorter.

The fourth part of each cycle is known as REM Sleep or rapid eye movement sleep. Our brain is fully active, and our eyes move even though the lids are closed. Our breathing quickens and our blood pressure rises. It is the period when we dream the most. It is also the time we lose the ability to move our limbs. Some suggest this is to avoid us enacting our dreams. In the first sleep cycle the REM sleep may be only ten minutes. It can rise to an hour in the last cycle of the night.

Our sleep cycles are remarkably consistent each night. Without being disturbed, we follow the same pattern of cycles and stages. It appears that we inherit this. Studies on twins show that the pattern is 50 to 75% inherited. Women sleep longer than men and have a higher proportion of SWA sleep.

Sleep and Memory

If you are sleep deprived, you will not remember things. The early explanations assumed that sleep stops us from being distracted while we were remembering. Modern theory suggests that sleep is a time when we consolidate memories. We encode memories throughout day. At night we need to “file” them. We need to integrate them into our existing store. We do this by first reactivating the memory of the event. We then consolidate it. We do this during REM and SWA sleep.

For many years it was thought that it was during REM sleep that the consolidation happened. During that period our brains were active, and we were dreaming. The link to dreams proved false. Most dreams do not involve reliving events of the day. Brain scans also show that it is in both SWA sleep and REM sleep that the reactivation and consolidation take place.

Reactivation was demonstrated elegantly by cueing the process. In one study smell was used. A pleasant smell was presented during a memory encoding exercise. This was while respondents were awake. Respondents played the card game of finding pairs in cards laid out face down. The same smell was then used that night when the respondents slept. By tracking their brain waves the smell could be administered during REM and/or SWA sleep. Recall was tested before sleep and the next day. The cards were laid out in the same pattern. Sleep improved the respondents scores. The respondents remembered much better when the smell was administered during REM and SWA sleep. With no smell or smell outside REM and SWA sleep their results were lower. Similar studies have used sounds to aid memory consolidation.

Ageing Sleep

In previous Newsletters I have talked about the changes in circadian rhythms that come with age. (Newsletter #049 Early Birds and Night Owls) . Older people are morning people. They start their days earlier and are at their cognitive best in the first few hours of the day. Doctors agree however that they need the same amount of sleep as their younger selves. Unfortunately, they tend to sleep less, many not reaching eight hours. Their nights are more disturbed.

The amount of SWA sleep an older person gets on an average night declines as they age. Their percentage of REM sleep stays approximately the same. This all starts around the age of 40. At the moment no one knows whether this a direct effect of ageing or the impact of disturbed sleep. This does affect their ability to consolidate their memory. It affects their episodic memory particularly. Their memory of “who”, “what”, “where” and “when”. Episodic memory decline is more common amongst the old. “Where did I park the car?” It is often attributed to lack of attention. That initial encoding of the memory may fail because of a distraction. It seems now that it may be due to the lack of the right kind of sleep.