The Hadza tribe live in the isolated areas of Northern Tanzania. They are hunter gathers. They have never adopted the “modern” idea of farming. They are living a life that characterized the whole of humanity at one time. They are window into our evolutionary past.
The Hadza operate in groups of 20 or 30. The group is mutually supportive in the hunting and gathering needed for their survival. Anthropologists have studied the sleep pattern of a group of Hazda. They used modern activity trackers to assess when the different members of the group were awake or sleeping. Over a twenty day period they found only eighteen minutes when the whole tribe was asleep. At any point of the day or night about a quarter of the tribe was awake.
The “sentinel hypothesis” points out that we are at our most vulnerable when asleep. When we all lived like the Hazda we needed to have guards or sentinels. They had to be able to stay awake at any time that people were asleep. Evolution thus created different chronotypes. People who are awake and active at different times and sleep at different times. We needed both “night owls” and “early birds” if we were to survive as a species.
Early Birds and Night Owls
People have different alertness / drowsiness rhythms. The early birds will have their peak alertness in the first five hours of their day. The Night Owl by comparison hates the morning. Their peak alertness time will be in the evening. This is not folklore. Scientists have been able to show a variation in everything from body temperature to melatonin production. There is in a spectrum of chronotypes. Each has a different rhythm.
We all might instinctively know our chronotype. For one group, knowing exactly when they are at their peak performance is crucial. Professional athletes want to be at their best at the time of the race or the game. Sports scientists do a lot of work on athletes, soccer players, basketball players etc. They need to understand when particular people will peak and the impact that has. Being at peak can improve performance by 2 seconds in a fifteen hundred metre race. It can cut seventy five seconds off a marathon time. That may not seem much but in the latest London Marathon that seventy five seconds was the difference between being 1st and 4th for men and 5th for women.
The Master Clock
Within our hypothalamus there are twenty thousand neurons which oscillate and are our master clock. This area of the brain is called the suprachiasmatic nucleus or SCN. Global standards of time are set by atomic clocks which keep time accurately over hundreds of years. They are carefully isolated from their environments. The SCN by comparison is continuously updating itself using outside stimuli. It is sensitive to light-dark cycles. It also adapts to our behaviour for example the time at which we eat.
The SCN controls peripheral oscillators that govern everything the body does. We have a clock for digestion and another one for sleeping and waking. We have clocks for core body temperature and hunger. There is even a clock for alertness. Part of the role of the SCN is to manage the alignment of the clocks.
The Ageing Clock
As we age the SCN is less effective at time keeping. The white matter or myelin that coats the interconnections between the neurons declines. So too does the production of neurochemicals. Two things happen. Older people are more likely to be morning people or “early birds”. Their clocks are moved forward by four or even six hours. After the age of sixty alertness and performance swings at different times of the day are bigger. Tested in the morning the sixty year old will perform the same as a forty or fifty year on most mental ability tests. They are as good at solving problems and their memory is strong. They are as good at reasoning and fine motor skills. Testing the same people in the evening or late afternoon and their performance drops compared to the forty to fifty year. By the age of seventy the differences are even greater.
The Aeon Mall in Tokyo I have described before. It is designed to serve the over 65’s. Interestingly it opens two hours earlier than most Malls. It opens at 7.00am and by 7.30 on most days is buzzing. The have special “golden hour” deals in many outlets. These are only available between 8am and 9am. They seem to know all about the ageing of the chronotypes.
It is important to make decisions in the morning if they are crucial. If you are crossing time zones there is even more to worry about. Our biological clock is much more flexible when we are young. It can adapt to changes in time zone more easily. Older people are more susceptible to jet lag. More specifically it takes them longer to recover. There is normal rule of thumb to ensure good decision making. You must allow one day to recover for each one hour time difference when travelling East. Coming home and flying West the same one hour time difference needs only half a day for recovery. As we get older resetting the clock takes longer. Older people are particularly susceptible to having to wake up earlier and go to bed earlier. Everything takes longer, sometimes much longer when travelling West.
The circadian rhythm means that most people have two energy dips a day. The first is in the middle of the night. The second is between one and three in the afternoon. As we age the post lunch dip in energy is a lot more noticeable. An afternoon nap is not a luxury but a necessity. All the more reason to make the important decisions in the morning.
Fixing the Clock to Sleep Well
Maintaining the stability of the SCN is important, especially with age. Older people need as much sleep as their younger selves. In many cases they just cannot get it and suffer disturbed nights. The arrival of artificial lighting changed forever the normal rhythms that the SCN used. Bright lights before bed are bad. It is even worse if the light is blue. We have more blue receptors in our retina than any other type. Somehow bright blue lights can disturb our normal rhythm and stop us sleeping well. Computers and smartphones before bedtime will upset everyone’s sleep. The Third Agers will have an especial problem.
The SCN reacts to our behaviours. It expects us to go to sleep at a certain time. If we want to sleep well, we had better do what it expects. A regular routine of going to bed and getting up helps us sleep. Eating also signals the SCN. If we eat within two hours of going to bed we create a disconnect between the SCN and the clocks in our stomach and intestines.