Most of us spend a third of our lives asleep, assuming that we average eight hours of sleep every night. We take sleep for granted and rarely spare a thought on the section of our lives spent in the arms of Morpheus. And yet, there are millions in the world who find it hard to drop off to sleep or go back to sleep when disturbed. Almost everyone experiences insomnia from time to time. Factors such as stress, jet lag, or even diet can affect your ability to get high-quality sleep. It is estimated that nearly 60 million Americans a year experience insomnia and wake up feeling unrefreshed. Sometimes the problem lasts for a night or two, but in other cases it is an ongoing issue.
General Practitioners are the first point of contact with patients and are crucial in detecting sleep deprivation and its consequent health effects. As such, it is desirable that they are updated on the Physiology of sleep, effects of sleep and sleep-related disorders. Likewise, doctors in hospital should be aware of the poor sleep quality in-patients are often subjected to in sleep-unfriendly hospital environments.
Sleep is a reversible behavioral state of unresponsiveness and perpetual dissociation from the environment. Consciousness is a perception of sensations; a state where voluntary initiation and control of movement, capabilities associated with higher mental processing are possible. Stupor is a state between consciousness and coma. Sleep is not a period of absolute rest; some parts of the brain are more active during sleep e.g. RL Stevenson wrote “Treasure Island” in his dream; the structure of the benzene ring he was trying to work out was worked out in a dream ( 6 monkeys dancing, holding hands).
Why do we sleep?
- Restorative (especially slow wave sleep)
- Conservation of energy
- Maintaining adequate function of the immune system
- Mentally practice & refine behavior e.g. escaping from predators without actually having to perform them
- Dreams occur during Rapid Eye Movement (REM) sleep, and we need dreams for mental health. Experiments where people have been awakened as they enter the REM sleep phase become moody & depressed, and disoriented; and may have hallucinations.We dream to forget ( “reverse learning ” helps us forget trivial things and to sort out our emotions)
Physiological changes during sleep
- Decreased reaction to external stimuli
- Reduced activity of the sympathetic system & parasympathetic system dominance
- Reduced heart rate, BP, respiratory rate (especially in NREM sleep)
- Reduced metabolic rate ( Somnolent Metabolic Rate or SMR is lower than BMR)
- Reduced muscle tone
- Extensor Babinski response (unmasking of extensor dominance)
- Changes in EEG
- Increased growth hormone production during deep sleep (decreased in REM sleep) helps in growth.
Fig.1 – Stages of sleep. Note that REM sleep is not entered in the first cycle.
[ Source: Principles of Human Physiology, CL Stanfield & WJ Germann, Pearson International, 3rd Ed. 2008 ]
In the first cycle, consciousness descends from stage 1-2-3-4, resurfaces from 3-2-1 and enters REM. In subsequent cycles, conscious level may reach 2 or 1, but always ends with REM. The cycles become shorter as the night progresses. In adults, sleep begins with NREM but with REM in infants. REM-onset sleep is seen in adults in jet lag, chronic sleep deprivation, narcolepsy, acute withdrawal of REM-suppressing drugs, endogenous depression.
Fig.3- Changes in sleep pattern with age.
Fig.4- Recommended hours of sleep for different age groups.
Regulation of sleep
- Sleep is regulated by an interaction of homeostatic and circadian processes.
- The homeostatic mechanisms keep track of how long we have been awake and asleep (how tired we are!)
- The circadian (circa=about; dian=day) process determines the optimal time for sleep.
- They influence sleep duration and the relative contribution of the two major types of sleep: non-rapid eye movement (NREM) and rapid eye movement (REM) sleep.
The suprachiasmatic nucleus (SCN) , also known as the “biological clock” dictates the sleep-wake cycles over a roughly 24-hour period; the SCN receives inputs via the retino-hypothalamic fibres , and is influenced by external cues (e.g daylight; light initiates sleep in nocturnal animals & waking in diurnal animals including humans), but the rhythm is still present in volunteers kept in caves or in people blind from birth. Melatonin & melanopsin may play a role [Phase shifts can occur with rapid crossing of time zones, resulting in ‘jet lag’]. This is an ultradian (many cycles within 24 hours) process that is characterized by the alternation of the two basic sleep states non-rapid-eye-movement (NREM) sleep and REM sleep influenced by SCN input.
Fig.5- Regulation of sleep
Hormones affecting sleep
In addition to melatonin, the hormones leptin, orexin, ghrelin are implicated in regulation of energy homeostasis and sleep/wakefulness. The levels of these substances exhibit circadian fluctuations.
Melatonin – Melatonin is a natural hormone produced by the pineal gland which is inactive during the day but becomes activated when the sun goes down. The pineal gland is switched on by the SCN and begins to actively produce melatonin, which is released into the blood. Melatonin levels in the blood stay elevated for about 12 hours and falls to daytime levels by about 9 am. Bright light directly inhibits the release of melatonin. Artificial indoor lighting can also be bright enough toinhibit melatonin release.
Leptin is produced by adipose tissue and is an appetite depressant. The levels normally rise during sleep, subduing the need to eat by reassuring the brain that energy reserves are adequate for the time being. Sleep deprivation lowers leptin levels in the blood whilst increasing ghrelin levels.
Orexin (also known as hypocretin) is a neuropeptide that regulates arousal, wakefulness, and appetite (feeding hormone). Lack of orexin is associated with sleeping disorders as well as in obstructive sleep apnoea (OSA). The most common form of narcolepsy associated with cataplexy, is caused by a lack of orexin in the brain due to destruction of the cells that produce it.
Ghrelin (“hunger hormone”) is an orexigenic hormone, but has many metabolic and anabolic effects in various parts of the human body. Its administration promotes wakefulness in rodents, while in human males it induces sleep ( but has no effect in women). Sleep deprivation increases ghrelin levelsIn addition to an important role in metabolism and appetite regulation it may participate in the energy balance during sleep. Plasma ghrelin concentrations are increased before meals and during the night.It acts on the hypothalamic-pituitary axis and other areas of the brain to stimulate a feeling of hunger and promote feeding.
Several reports affirm that when the average duration of daily sleep is less than 7.7 hours, individuals of all age groups show a higher mean BMI, as well as leptin reductions and ghrelin increases that are independent of age and sex. These hormonal changes are similar to conditions like poor energy intake or weight loss, when the body adapts byan increase in eating. This is a biologically plausible mechanism for the increased BMI that accompanies poor sleep.
Sleep Deprivation
Sleep deprivation is marked by brief psychosis but non-permanent psychological effects viz irritability , stimuli misperception, decreased waking alpha activity, disorientation, lack of attentiveness Neurological consequences are seizures and tremors
Fig.6- Consequences of sleep abnormalities
Sleep disorders
Somnambulism, nocturnal enuresis (bed-wetting) & night terrors occur during arousal from SWS. Nocturnal emissions (‘wet dreams’) are normal in males of reproductive age & even in females
Sleep Apnoea is stopping of breathing for short periods (typically between 10 and 90 seconds) during a normal nightly sleep. It can be repeated hundreds of times during a sleep period, and is nearly always accompanied by snoring, which is usually quite loud. The patient is unaware of this happening, in normal circumstances. There are three types of Sleep Apnea .(1) Obstructive sleep apnoea (OSA) – Fairly common; upper airway collapses as the throat muscles relax during normal sleep ; cerebral hypoxia results in waking up; may be repeated every 15 minutes; excessive daytime tiredness and a lack of concentration ; relieve the obstruction (sleep on the side; reduce weight) (2) Central – Rare; neurological dysfunction, the brain simply “forgets” or “delays” to send the signal to breathe; more difficult to treat; some drugs may help (3) Mixed – Extremely rare.
Insomnia
Insomnia regularly affects millions of people worldwide. The effects can be devastating, and may be in the form of daytime sleepiness, lethargy, a general feeling of being unwell, both mentally and physically, mood swings, irritability, and anxiety. It has also been associated with a higher risk of developing chronic diseases. According to the National Sleep Foundation, 30-40 percent of American adults report that they have had symptoms of insomnia within the last 12 months, and 10-15 percent of adults claim to have chronic insomnia. The types are:
- chronic insomnia, lasting a month or longer
- acute insomnia, lasting a day or days, or weeks
- comorbid insomnia, associated with another disorder
- onset insomnia, difficulty falling asleep
- maintenance insomnia, inability to stay asleep
- waking up early in the morning is a red flag for depression,
There is sometimes an underlying medical condition that causes chronic insomnia, while transient insomnia may be due to a recent event or occurrence. Insomnia is commonly caused byphysical and psychological factors including
- Disruptions in circadian rhythm – jet lag, job shift changes, high altitudes, environmental noise, extreme heat or cold.
- Psychological issues – bipolar disorder, depression, anxiety disorders, or psychotic disorders.
- Medical conditions – chronic pain, chronic fatigue syndrome, congestive heart failure, angina, acid-reflux disease (GERD), chronic obstructive pulmonary disease, asthma, sleep apnoea, Parkinson’s and Alzheimer’s diseases, hyperthyroidism, arthritis, brain lesions, tumours, stroke.
- Hormones – oestrogen, hormone shifts during menstruation.
- Other factors – sleeping next to a snoring partner, parasites, genetic conditions, overactive mind, and pregnancy.
Management of insomnia includes identifying the cause, appropriate behavioural changes and medication.
Hospitals are sleep-unfriendly
The hospital environment can be very sleep-unfriendly as the body’s needs and circadian rhythms that regulate our sleep/wakefulness, functioning of the immune system and hormones are largely ignored. The bright lights and intense activities in hospital are not optimal for patient recovery. Sadly, our culture does not emphasize on speaking softly in public places and healthcare personnel often speak or laugh loudly ; coarse language is also not avoided in some hospital settings. It is time our personnel recognise the importance of good sleep quality in the management of patients.
References
- Insomnia: Health Effects, Factors, and Diagnosis – Healthline https://www.healthline.com/health/insomnia-concerns (http://www.circlecity.co.uk/sleepdesk/sleep_apnoea.php#
- Principles of Medical Physiology by S.SircarThieme NY. 2008, pp 692-696
- Review of Medical Physiology by W.F.Ganong , Lange , Chapter 11
Sleep& circadian rhythms - The Physiology of Sleep Deprivation: Inadequate Rest can Have Profound Metabolic
Consequences Stephen Allen Christensen http://sleep
disorders.suite101.com/article.cfm/the_physiology_of_sleep_deprivation
