The science of sleep
Sleep science is still a relatively new area of research and to this day, scientists struggle both to define sleep, and to agree on its core function (Kirsch 2011).
Great strides have been made towards understanding the processes involved in sleep over the last decade however, and advancing technologies continue to allow us to monitor and study sleep in depth.
The history of sleep research
Modern research into sleep is said to have begun with an American scientist, Dr. Nathaniel Kleitman.
Dr. Kleitman was the first person to focus research solely on the science of sleep, founding the first ever sleep laboratory at the University of Chicago, and publishing his findings for those interested in the field to learn from.
Progress in sleep research was later made by Dr. William Charles Dement, one of the first scientists to measure brain activity during sleep using electroencephalography, or 'EEG'. This allowed the stages of sleep, or 'sleep architecture' to be observed, and subsequently defined, for the first time.
What is sleep?
Carskadon and Dement (2011) define sleep as a “reversible behavioral state of perceptual disengagement from, and unresponsiveness to, the environment”.
It is possible to divide sleep into two, broadly defined, states: rapid-eye movement (REM) sleep and non-REM (NREM) sleep. We normally enter sleep through NREM sleep but alternate between NREM and REM sleep throughout the night. This process is known as a 'sleep cycle'.
Sleep cycles tend to last approximately 90 minutes, with good sleepers generally going through 4-5 sleep cycles on an average night. The composition of these cycles changes throughout the night, with the first third of the night tending to be made up of greater amounts of slow-wave (deep) sleep. In contrast, towards the end of the night it is REM sleep which dominates, followed by changes in body temperature.
The distribution of average time spent in specific sleep stages throughout the night is as follows:
• Wakefulness (5%)
• Stage 1 (2-5%)
• Stage 2 (45-55%)
• Stage 3 (3-8%) [slow-wave sleep]
• Stage 4 (10-15%) [slow-wave sleep]
• REM sleep (20-25%)
Sleep assessments, usually carried out in sleep laboratories or sleep centers, include three different types of measurement:
• Electrical activity in the brain is measured by electroencephalography (EEG), is used to differentiate between wakefulness, sleep, and the different stages of sleep.
• Muscle activity is measured using electromyography (EMG), because muscle tone also differs between wakefulness and sleep.
• Eye movements during sleep are measured using electro-oculography (EOG). This is a very specific measurement that helps to identify Rapid Eye Movement or REM sleep.
This system of assessment is referred to as polysomnography, or 'PSG'.
Carskadon, M.A., Dement, W.C. (2011). Monitoring and staging human sleep. In Kryger, M.H., Roth, T., Dement, W.C. (Eds.), Principles and practice of sleep medicine, 5th edition, p16-26. St. Louis: Elsevier Saunders.
Kirsch, D.B. 2011. There and back again: a current history of sleep medicine. Chest, 139(4), 939-946.