Sleep and timing. Does it matter when we sleep?

Maria M. Hadjimarkou, PhD. Lecturer School of Psychology University of Sussex

Timing is everything, some say. We try to fit in as much as possible in our busy lifestyles. We each have massive to-do lists, trying to be productive, accomplish things and feel good. Often, sleep is squeezed to only a few hours at night. We may postpone sleep until we finish a project or finish watching our favourite show. We can sleep the next day if necessary, right? Well, this is not exactly the case. There is a right time to sleep and a right time to be awake.

All living organisms on Earth have a natural rhythm, which is regular and predictable. The rhythms we all display are called circadian rhythms, which refers to circa and dies from Latin, meaning ‘about a day’. Earth rotates on its axis and we are exposed to the sun in regular, predictable intervals. All living things have evolved to work around a more or less a 24h period. Some of the circadian rhythms can be readily observed, like the opening and folding of plant leaves like the mimosa, or sleep and wakefulness in animals. Others are not so easy to observe, such as the expression of genes and the cellular events in our bodies.

Wakefulness naturally takes place in the morning with the emergence of light whereas sleep naturally follows at night. We usually feel alert during the earlier part of the day and as the day progresses, we start to feel less alert and eventually we feel sleepy. This change in our activity is partly driven by a homeostatic drive, internal signals that tell us that we need to slow down and sleep, and partly by a circadian drive which takes into consideration the time of day and the presence or absence of light in our environment.

We know that our sleep and wakefulness have evolved to take place at a particular time because our biology tells us. Deep in our brains, we each have a biological clock, a brain area whose molecular components are functioning on a 24h-period. This area is called the suprachiasmatic nucleus (SCN), which communicates with dozens of other areas in the brain, to signal activity or quiescence. The neurons in the SCN function in a rhythmic fashion; they are active during the day and quiescent during the night. Even if the SCN neurons are removed from the brain and they are placed in a dish, their activity continues to be regular for a long time. This rhythmic activity is communicated beyond the brain, to the different organs in our body, which in turn adjust their activity based on the input from the SCN.

Besides its intrinsic activity, the SCN is also constantly receiving information about how much light is out there from our eyes. The detection of light is made possible by a special group of cells in our eyes, which are different from the ones required for vision. This may also explain why people who are blind are still aware of the time of day and display rhythmicity. These cells respond to the changes in light intensity and communicate this information to the SCN.

Light is the most important signal or zeitgeber, ‘time-giver’ in German, so when the SCN responds to light it sends signals to a small gland in the centre of our brain called the pineal gland. When the day is coming to an end and the night starts to fall, our pineal gland starts to secrete melatonin, a hormone that promotes sleep and other functions that take place during the night. The presence of light inhibits or delays the secretion of melatonin.

Our bodies anticipate day and night because they have developed these mechanisms through evolution. Our brain can anticipate morning wakefulness, which is why right before we wake up in the morning our temperature rises, our cortisol levels and our blood pressure increase so that we can get up and start our day. As the day comes to an end, we tend to wind down and sleep in order to allow restoration. Or do we? In previous centuries, this was a very straightforward task. In modern times and the last hundred years or so, things have become rather complicated, because technology and access to light and stimulation have totally changed our lives. We can prolong the daytime by just switching on the lights in our house. We can continue to work until the early morning hours because of electricity and technology. Can we just prolong the daytime and sleep will eventually come? What about those that work night shifts, who are regularly turning night into the day because they have to work?

Shift work is causing havoc in our bodies and renders shift workers vulnerable and prone to developing several health conditions including sleep problems, gastrointestinal problems, hormonal dysregulation, mental health difficulties and even cancer (see picture). Despite all the efforts to achieve reverse entrainment by using bright lights at work and dim lights at home, the vast majority (97%) of shift workers are not able to adjust to a reverse cycle. Even when different combinations of day and night shifts were tried, it was not possible to reverse the ill effects of this mismatch between our biological clock and the environment. It is not possible to change the way that our bodies have evolved. These findings extend well beyond shift workers who most likely don’t have a choice about their work schedule. Many of us voluntarily disrupt our natural processes and health, by exposing ourselves to light at night. Light in turn delays the secretion of melatonin which interferes with several functions that take place at night and eventually delays or even disrupts sleep.

Sleep is meant to take place at night, that is what our bodies have evolved to do. Because we can have access to light when we please, we have developed a sense of empowerment, a sense that we have surpassed our bodily restraints. Nothing is far from the truth. Research evidence is in fact hambling. Our bodies are not happy to jump on board with our developments in technology and a haphazard switch across different vigilance states. Timing may indeed be everything and harmonising our daily schedules with our bodily functions and the natural environment is crucial for maintaining our health and well-being. Being active during the day and going to sleep at night, is the best way to make the most of our time awake and get the best sleep possible when we are in bed. Our bodies demand respect.

Key Reference: Foster RG, Wulff K. The rhythm of rest and excess. Nat Rev Neurosci. 2005 May;6(5):407-14.