Sleep. It’s necessary for all life, from humans to insects, mammals and reptiles. Every living species sleeps, but why is it important? Sleep affects every aspect of our well being. You can argue that it is not just a pillar of our health but rather the foundation.
Sleep enhances learning and memory, boosts our immune system, prevents infection, malignancy and other diseases, reduces heart disease, regulates appetite and metabolism, improves athletic performance and recovery, inspires creativity and wards of mental illness.
Our body has two systems that influence our sleep – wake cycle: an internal, biological clock (circadian rhythm) and a chemical substance called adenosine (sleep pressure).
- Circadian rhythm
Like the beat of a drum, this rhythm repeats itself roughly every ~24 hours. It is controlled by an area in the brain called the suprachiasmatic nucleus. Natural light from the sun provides a constant signal to our brain to synchronize this rhythm. Melatonin is a hormone secreted by the brain into the bloodstream and helps signal to the body that it is time to sleep.
While circadiam rhythm is the strongest signal, it is not the only signal that our brain can use to reset our biological clock. Food, exercise, temperature, social activities or any repeating stimulus can be used to influence our biological clock.
- Sleep Pressure
You know that unavoidable sensation of sleep where you are overcome with fatigue and can’t keep your eyes open? This is caused by a substance called adenosine, which builds up in your brain with each passing minute you are awake and is a byproduct of the breakdown of adenosine triphosphate (ATP), a molecule used for energy production. Adenosine binds to receptors in the brain but caffeine can also block these adenosine receptors, giving a false feeling of alertness, while the concentrations of adenosine continue to rise. This eventually leads to the caffeine crash where a flood of adenosine binds to receptors and leaves you feeling tired.
These two factors are independent but synchronized to manage the sleep wake cycle. The urge to sleep increases at night when adenosine builds and circadian rhythm drops. But when circadian rhythm and sleep pressure are closest in mid-late morning, a feeling of wakefulness and alertness occurs.
There are two phases of sleep: rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. Both phases have very distinct brainwaves. If you were to translate REM sleep brainwaves into sound, it would be chaotic, similar to placing a microphone in a large stadium where everyone is having separate conversations. On the other hand, the brainwaves of NREM sleep are more cyclical, similar to synchronized chanting from a large crowd.
Our sleep is divided into 90 minute cycles where the ratio of REM to NREM sleep changes throughout the night. The graph produced from these cycles of brain wave activity is called a hypnogram. The first half of the night is predominantly composed of NREM sleep. One of the key features of NREM sleep is to remove unnecessary neural connections. NREM stage two sleep is important for memory and motor skill development i.e. playing the piano) especially during the last 2 hours of sleep. On the other hand, REM sleep is crucial to form new neural connections, integrating past and present memories to form new insights and creativity. If you delay your bedtime by a few hours, you are missing out on NREM sleep. The same is true if you wake up earlier than normal, except this time you lose a larger proportion of REM sleep.
The rest of this series explores how sleep impacts our health and provides tips for better sleep hygiene.