Overview
- Sleep is a recurring, active, complex neurophysiological process of the brain that is necessary for survival and recovery. Human sleep is composed of two distinct states: rapid-eye movement (REM) and non-rapid eye movement (NREM) sleep.
- Stage N1 brain activity is less than in relaxed wakefulness, but is higher than the other sleep stages.
- The most prominent characteristics of N2 sleep are sleep spindles and K-complexes.
- N3 is considered to be deep sleep, or slow-wave sleep, and the restorative stage of sleep.
- REM sleep is characterised by rapid eye movement.
- The homeostatic sleep drive and the circadian process, are involved in the control of the timing and organisation of sleep.
- Problems with sleep are associated with a reduction in cognitive factors including work performance and an increased risk of motor vehicle accidents. There are also adverse effects of inadequate sleep on the human body and brain.
- Healthy sleep can be trained and regulated!
What is Sleep?
Sleep is a recurring, active, complex neurophysiological process of the brain that is necessary for survival and recovery. Human sleep is composed of two distinct states: rapid-eye movement (REM) and non-rapid eye movement (NREM) sleep. NREM sleep is divided into three stages (N1, N2, N3), which are categorised by synchronous cortical neuron activity, minimal psychological activity, and stable autonomic function i.e stable involuntary body functions, such as heartbeat, blood flow, and breathing. (Luyster et al., 2012). REM sleep (N4) typically occurs after N3 and is characterised by rapid eye movements (hence the name), dreams, and an increase in cortical neuronal activity. When you fall asleep, you start in stage 1 and slowly progress through stages 2 and 3. After roughly one hour of sleep, you will begin to cycle back from stage 3 through stages 2, 1 and then REM. This sequence repeats between four to six cycles in an average nights sleep, with each cycle lasting typically between 90 and 110 minutes (Luyster et al., 2012).
Stage 1 (N1)
During the first stage of NREM sleep, your heartbeat and breathing will slow down, your muscles may suffer from sudden jerks and spasms, and you may feel the sense of falling. Stage N1 brain activity is less than in relaxed wakefulness, but is higher than the other sleep stages. This stage of sleep lasts for approximately 5-10 minutes.
Stage 2 (N2)
During the second stage of NREM sleep, your slow eye movements will stop, your breathing and heart rate will slow even more, and your body temperature will begin to drop. The most prominent characteristics of N2 sleep are sleep spindles and K-complexes. Sleep spindles consists of 12-14 Hz waves during a burst that results from the oscillating interactions between the cells in the thalamus and the cortex. A K-complex is a sharp wave that is associated with the temporary inhibition of neuronal activity (Cash et al., 2009). This stage is considered to be light sleep and lasts for approximately 10-25 minutes.
Stage 3 (N3)
During the third stage of NREM sleep, your brain activity slows down and becomes synchronous, where slow and large-amplitude waves become more common. During this stage, sensory input to the cerebral cortex is greatly reduced, which results in neurons synchronising their activity. Your heartbeat and breathing will be at its lowest of the day, and it is during this period when muscle tissues will repair and regrow. This stage is considered to be deep sleep, or slow-wave sleep, and the restorative stage of sleep. This stage lasts for approximately 20-40 minutes.
Stage 4 (REM)
During the fourth stage of sleep, your eye activity will increase significantly, hence why this stage is called rapid-eye movement (REM) sleep. Brain activity increases to almost wakefulness activity, heart rate and blood pressure increase, and dreams and memories form. Your body experiences a temporary paralysis of the muscles, with two exceptions: the autonomic muscles that control breathing, and the eyes. Under normal circumstances, you will not enter a REM sleep stage until you have been asleep for 90 minutes, and at its first cycle will only last for up to five minutes. As more cycles of sleep are completed, REM sleep will last longer, where in its final cycles it may last up to one hour.
Why Does Sleep Occur?
A fundamental homeostatic process is involved in the regulation of sleep. This homeostatic pressure to sleep increases as a person stays awake; the longer you stay awake, the stronger the drive to sleep becomes (Luyster et al., 2012). Individuals with sleep deficiency due to short sleep duration accumulate a sleep debt, which will result in shorter sleep latency, greater total sleep time, and enhancement of EEG synchrony in NREM sleep (Luyster et al., 2012). Even when an individual is active and resisting sleep, if sleep debt is too great, transitions into brief periods of sleep will occur. These brief periods are called “microsleeps”, and generally last from 3 to 30 seconds and occur without awareness of the individual (Luyster et al., 2012).
In addition to the homeostatic sleep drive, a second process, called the circadian process, is involved in the control of the timing and organisation of sleep. The brain region responsible for this process is the suprachiasmatic nucleus. The human circadian process incorporates 3 different components (Luyster et al., 2012): (1) Input pathways that transmit light and other signals to the circadian “clock” and thus entrain circadian rhythms to environmental cues, primarily the light-dark cycle; (2) an endogenous circadian pacemaker that generates rhythms having a period of approximately 24 hours; and (3) output pathways controlled by the pacemaker. In the absence of environmental cues, circadian rhythms are intrinsic and will continue to oscillate with a period of approximately 24 hours (Luyster et al., 2012).
These two systems work together. The gradual build-up of sleep debt across the day is opposed by incrementally increasing brain alerting signals, which rather abruptly dissipate around the usual time of sleep onset at night and “permits” sleep to occur (Luyster et al., 2012). Without the opposition of the circadian system, humans would be unable to sustain wakefulness for the usual 16 hours each day because of the growing homeostatic sleep pressure. The evolution of two temporally and precisely complimentary processes which maximize daily sustained periods of both wakefulness and sleep highlights essential biological need for sleep (Luyster et al., 2012).
Problematic Sleeping And Major Health Risks
Despite its critical importance, it is reported than up to 35% of adults in Australia suffer from inadequate sleep, whether its duration or its quality (Adams et al., 2016). In Australia, the average reported daily amount of sleep is seven hours, with up to 12% of people sleeping less than five hours a night (Adams et al., 2016). This inadequacy of sleep is a serious issue. While experimental studies of prolonged sleep deprivation cannot be conducted in humans, growing evidence over the last decade suggest that habitually shorter sleep duration is associated with greater mortality (Luyster et al., 2012). Problems with sleep are associated with a reduction in cognitive factors including work performance and an increased risk of motor vehicle accidents (Adams et al., 2016). There are also adverse effects of inadequate sleep on the human body and brain. Short sleep affects the neuroendocrine system, resulting in an increased risk for obesity, type 2 diabetes, heart disease, infections and stress. Problematic sleeping is also linked to disturbances in psychological factors including mood, memory, concentration and learning (Adams et al., 2016).
Is it possible to learn to sleep better?
Yes, it is possible! Hauri (1991) developed a set of behavioural and environmental rules that are designed to promote good, restful sleep, called sleep hygiene. Healthy sleep can be trained and regulated. Here are top 5 recommendations for healthy sleep hygiene from the UCSD Center for Pulmonary and Sleep Medicine (2017):
1. Don’t go to bed until you’re sleepy.
2. Avoid caffeine if possible.
3. Avoid alcohol if possible.
4. Do not smoke cigarettes.
5. Try to get up at the same time every morning.
If you want to hear further about how to improve your sleep, watch the video below! Dr Matthew Walker is a professor of neuroscience, with his area of expertise in sleep. He highlights the dos and don’ts of sleep and how best to improve it.
References
Adams, R., Appleton, S., Taylor, A., McEvoy, D., & Antic, N. (2016). Report to the sleep health foundation 2016 sleep health survey of Australian adults. The Adelaide Institute for Sleep Health.
Cash, S. S., Halgren, E., Dehghani, N., Rossetti, A. O., Thesen, T., Wang, C., … & Ulbert, I. (2009). The human K-complex represents an isolated cortical down-state. Science, 324(5930), 1084-1087.
Hauri P.J. (1991) Sleep Hygiene, Relaxation Therapy, and Cognitive Interventions. In: Hauri P.J. (eds) Case Studies in Insomnia. Critical Issues in Psychiatry (An Educational Series for Residents and Clinicians). Springer, Boston, MA. https://doi.org/10.1007/978-1-4757-9586-8_5
Luyster, F. S., Strollo, P. J., Zee, P. C., & Walsh, J. K. (2012). Sleep: a health imperative. Sleep, 35(6), 727-734.
Featured Image Credit Zelenov Iurii/Shutterstock
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