Sleep: Regulation and Assessment
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Introduction[edit | edit source]
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Regulation of Sleep
Two independent, yet interrelated processes govern sleep with regards to the:
Sleep timing
Sleep intensity
Sleep duration.
These two processes are:
Homeostatic Sleep Drive
Circadian Rhythm
Homeostatic Sleep Drive
Also known as process S
Represents sleep debt
Sleep drive or the need for sleep increases during wakefulness and decreases during sleep
Regulated by the duration of which we have been prior awake
The accumulation of sleep-inducing substances in the brain – increasing levels of the hormone Adenosine
Adenosine starts to build up from the moment of wakefulness and only relieved to the point of which one falls asleep
Adenosine augments sleep propensity (the natural tendency to want to fall asleep)
Caffeine is a stimulant that binds to the Adenosine receptor and blocks the impact of the desire for sleep propensity, but the pressure to sleep will still build up nonetheless. In a sleep deprived state the homeostatic drive for sleep will increase and sleep will most likely occur at inappropriate times or unusual times
Typical phenomenon in a sleep-deprived individual is “micro-sleep” (when a person will disengage from their physical surroundings due to a momentary lapse in visual processing. Sleep debt can be paid back, but the long-term effects of sleep deprivation can be detrimental to physical, mental and emotional health. Read more about Sleep Deprivation here.
Slow wave sleep (SWS) is the principal marker of homeostatic sleep drive
Slow wave sleep – greatest during initial sleep periods when the desire for sleep is very high, this decreases as sleep is initiated. For example, taking a nap during the day may decrease the level of deep sleep.
Therefor slow wave sleep activity and the measurement thereof represents an important marker of the homeostatic process
Circadian Rhythm
Also known as process C or the sleep/wake cycle
Most predominant of biological functions involved in the normal sleep and wakefulness cycle
Dependent on system which oscillates within a period of approximately 24 hours, with daily alterations between lightfulness and darkfulness
Human circadian rhythm is actually more than 24 hours – but light assists with regulating it into a 24 hour model
Superchiasmic nucleus is the circadian pacemaker or biological clock in humans
SCN is situated in the hypothalamus
SCN is synchronised by exogenous environmental cues known as zeitgebers (time-giver)
Most powerful zeitgeber is light – this activates photoreceptors in the retina and this inhibits the secretion of melatonin by the pineal gland
Sleep propensity of humans corresponds to body temperature and specifically nadir
Nadir is the point where the lowest core temperature is recorded
Body temperature cycles are also controlled by the hypothalamus
Body temperature increases during the day and decreases during the night
These temperature peaks and troughs are hypothesised to mirror sleep rhythm
Maximum alertness will occur near the peak body temperaturehttps://www.medscape.com/answers/1188226-194382/what-are-circadian-sleep-rhythms#:~:text=Circadian%20sleep%20rhythm%20is%20one,rhythms%20modulated%20by%20the%20hypothalamus.&text=The%20nadir%20of%20this%20rhythm,restoration%20by%20preventing%20premature%20awakening.
Drowsiness will increase as temperature decreases
Circadian nadir will be when sleepiness become overpowering (typically around 2-3 am in the morning – people doing shift work find it difficult to stay awake around this time)
Cycles between sleepiness and alertness at regular intervals
Two -process model of sleep - interaction
This model proposed by Borbely posits that the ability to initiate sleep is determined by both the homeostatic sleep drive and the Circadian process. The homoestatic process, which is derived from the time course of slow wave activity, is represented by a linear increase during wakefulness, and a linear decline during sleep. In ideal, non-sleep deprived scenarios the interaction between the homeostatic sleep drive and the circadian process is relatively synchronous.
Daan et al investigated the timing of sleep (onset and duration) and determined and developed upper and lower thresholds to predict sleep timing. Taking a nap during the day will effect the consistency in sleep patterns and causes a transistory depression of the upper threshold of the homeostatic process. The evening maintenance wake zone refers to the increased level of self-alertness before falling asleep. This indicates the circadian impact on sleep propensity. In the early evening before bedtime the circadian drive for sleep is very low, shortly before the onset of melatonin secretion. Although, this may seem counterintuitive, as it implies a high circadian drive for alertness in the evening, it was shown that the circadian arousal signal in the early evening causes higher subjective and objective alertness and it actually opposes the accumulated homeostatic sleep drive. This dynamic interaction between these two processes in sleep/wake regulation allows for a wake period of around 16 hours during the daytime and a consolidated sleep period of approximately 8 hours at night, in humans.
External conditions affecting the threshold levels
Sleep deprivation conditions create a suspension in the upper thresholds, therefore allowing process S or homeostatic sleep drive to increase further
Other factors such as bed rest, warmth, darkness or the absence of social stimulation lowers the upper threshold so that sleep is precipitated.
The circadian rhythm modulate these two thresholds and it determines the onset and termination of sleep episodes. Johns et al developed a further process stating that sleep and wakefulness at a particular time is dependent on the relative strength of both processes but not on the absolute strength of them. It is theorised that the secondary wake drive is influenced by things such as
Posture
Behaviour
Physical activity
Feelings
Mental activity
This then supports psychological approaches which have been shown to be effective in treating sleep related disorders like insomnia.
The magnitude of the secondary sleep drive may be the most important determinant of sleep propensity as changes in this can be controlled by individuals themselves. This then emphasis that people need to look after their mental and physical health, and why sleep plays such a key role in the management of chronic pain.
Factors implicated in sleep propensity
Time of day
Previous sleep deprivation
Medication and their effect on the CNS
Age
Physical state of an individual
Cognitive state of an individual
Irregular work hours, shift work
Presence of a sleep disorder
Sleep deprivation and appetite
During stages of sleep deprivation, an increase in appetite is noticeable during the increased levels of sleepiness. This is due to changes in two of the main hormones which governs the level of hunger. These hormones are:
Leptin – hunger decreasing hormone
Ghrelin – hunger increasing hormone.
Studies have shown that individuals who works against their natural circadian rhythm are at an increased risk of obesity, cardiovascular disease, etc.
Polyphasic Sleep
Initially sleep was regarded as monophasic in nature, but anthropologically it is viewed as a polyphasic system with at least two phases of position for the occurrence of sleep in a 24 hour cycle. The lesser known phase is the one around the postprandial dip (term used to refer to mild hypoglycaemia occurring after ingestion of a heavy meal) after lunch time.
In Mediterranean countries, siestas or a shorter period of sleep during the day is still a common occurrence and the health benefits of their diets and lifestyles can teach us a lot.
Monophasic sleep
“normal” sleeping period in today’s society
Individual sleeps once per day, typically between 7 -9 hours a night
This sleep pattern was influenced by the industrial revolution;s longer-than-normal working hours. Other theories argue that the arrival of electricity and the increased exposure to bright lights caused a drop in melatonin levels and this could have negative impact on sleep duration
Biphasic sleep https://www.medicalnewstoday.com/articles/319425#types-of-sleep-patterns
Typically, people sleep for a long-duration at night (5-6 hours) with a shorter period of sleep or siesta during the day (this period typically lasts about 30 minutes and is seen as an energy booster). Siestas can however last for longer, up to 90 minutes, and this allows for an individual to have one complete sleep cycle.
Another form of biphasic sleep is segmented sleep. This includes two sleep periods, both at night.
Polyphasic sleep
Polyphasic sleepers rest between 4 -6 times a day. Various sleep combination categories exist. These are:
Everyman – long sleep of approximately 3 hours with approximately three 20-minute naps throughout the day
Uberman – only 3 hours of sleep per day in the form of six 30 -minute naps throughout the day
Dymaxion – Only 2 hours of sleep per day in the form of 30 minute naps every 6 hours
Measurement of Sleep
To promote effective behavioural change in sleeping patterns in ourselves and our patients it is important to have a good awareness of the level of sleepiness. Insomnia is likely to be prevalent in many healthcare workers and patients. Insomnia does not just relate to the inability to sleep, but also to the inability to stay asleep or an increased incidence of early wakefulness.
Circadian Rhythm
Sleep can be measured objectively and subjectively.
Objective measures
Objective measures provide a measure of sleep latency in sleep laboratories during the day. These types of tests are reckoned to be the gold standard in the assessment of specific sleep-related outcome measures. One test that is commonly used is the Multiple Sleep Latency Test (MSLT). Another test that is less used is the Maintenance of Wakefulness Test (MWT).
Multiple Sleep Latency Test
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5971842/
Test to identify excessive daytime sleepiness (such as feeling sleepy in situations where one should be awake or alert, for example driving a truck)
Determines how long it takes an individual to fall asleep
Identifies the phases of sleep (how quickly and often an individual enters REM sleep)
Provides an indication of sleep propensity against the polysomnogram
Standard test to diagnose idiopathic hypersomnia and narcolepsy
Measures how quickly an individual fall asleep during the day in a quiet or non-stimulating environment
Often the MSLT starts the morning following a Polysomnogram (PSG) and lasts one complete day
With this test the individual tries to sleep in five scheduled naps, separated by two-hour breaks
Test is often also called a “nap study”
Each trial is situated in a quiet bedroom/ area
Individual is connected to device that detects sleep stages and procedure includes EEG, EOG, EMG and EKG
MSLT can identify exactly when an individual fall asleep and if the individual entered REM sleep
If individual fall asleep, they are awakened after 15 minutes
Nap trial also ends if individual does not fall asleep within 20 minutes
Individuals with narcolepsy – often have two or more REM periods during he MSLT
Individuals with idiopathic hypersomnia – easily fall asleep, but do not reach REM sleep during nap trial
Maintenance of wakefulness test (MWT)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5971842/
test is performed over a whole day
test is conducted while individual is awake
instructs individuals to remain awake rather than fall asleep during periodic tests
MWT helpful in management of sleepy patients – especially for driving purposes
Measures how alert an individual is during the day
Determines if an individual can remain awake for a period in a quiet, relaxing and non-stimulating environment
During the test – four to five periods of approximately 40 minutes each, with two-hour breaks in-between, the individual is asked to stay relaxed in a quiet, faintly-lit bedroom
First trial usually starts 1.5 to 3 hours after the individual’s normal wake-up time
Individual has breakfast one hour before the first relaxing period, and lunch after the second period
Between test periods, individual can read, watch TV, have a meal and move freely inside the test building.
Individual can not go outside during the test as daylight is a factor that must be eliminated during the test
During relaxing periods – individuals connected to set of leads that monitor
Heart activity (2-3 ECG leads)
Brain activity (4 EEG leads)
Chin muscle activity (3 leads)
Left and right eye movements
Test is terminated if individual fall asleep for 90 seconds at any time during the relaxing period
Sleep specialist analyses the data to determine the individual’s level of sleepiness during the day
Psychomotor Vigilance Task
Another way to objectively assess level of sleepiness
Well-validated measure of neurobehavioural alertness in sleep research
Used to quantify the response to sleep loss through measuring the ability to sustain attention and promptly respond to salient signals
PVT -requires response to a stimulus (digital counter) by pressing a button as soon as the stimulus appears, this stops the stimulus counter and reaction time is displayed
Accurate measure of neurobehavioral performance as it is
indicative of a fundamental aspect of waking cognitive function
easily performed and administered
minimally affected by learning/aptitude
brief
valid, reliable and sensitive
The advantages of these methods include:
The use of advanced technology that cannot be used at home
Precise and discrete methods – can distinguish between different sleep phases
Gold standard for sleep evaluation
The disadvantages are:
Expensive methods
Time-consuming methods
Labour-intensive measures
Require professional assistance
Can only be done over a short period of time (one or two days)
Another important functional disadvantage is that the assessment in a sleep laboratory is not done in the usual seep context of the patient at home, therefore it does not really measure a normal sleep situation.
Sleepiness and Safety
Simulated environments are good predictors of levels of sleepiness. Assessment of an individual’s performance (how much they perform and how well they perform) in a non-sleep deprived state versus a sleep deprived state will provide information of the impact of sleep deprivation on performance. Keep in mind that in some performance assessments there is a significant learning curve effect and that needs to be considered when research is done in the area of sleep deprivation and performance. (course and https://watermark.silverchair.com/sleep-28-12-1511.pdf?token=AQECAHi208BE49Ooan9kkhW_Ercy7Dm3ZL_9Cf3qfKAc485ysgAAArgwggK0BgkqhkiG9w0BBwagggKlMIICoQIBADCCApoGCSqGSIb3DQEHATAeBglghkgBZQMEAS4wEQQMWIofBpaBKJ0wqxsSAgEQgIICa4_5ptoPUk96xo52JxindPpd0h3YXyTp1erqtKpVhqhcZSsA3FhoSMx_21VXF_fNXg-OMjXDa4mqIZIIwhz2e_jhiwrFDiB570I1pBHahvJrJb6nOWTOX6ENEa2UcT41DNWbtUyI6xxSONJtWkYKTUnkLakS1eOg8PwhNml8kjjjv6RaFXp62vODow1ascTxxVrJ9N_Rs_5SiyiMJnm-uQO8_wXTiJjaVPc46IOv827TLBjOucDezzSmMDoMK9aCQh68ImuJb788RjvDpxrFbyHvfKR9gHNK4HJxtBUizCOJNsRoCQicYvqFkfAdwKX6Ry6v9wrr8HSuS79fdQfReHo_jZD_q_P609-hfujxetZaEn_qm5bvLGM3utEgr8t9yDQsAlxX5J0AicyuvsomBWhVw71lywTbnJtLBh9S3vNFUTs1yRPucFF5L7Z62zg9HRUBg82LqJOM7NrCdTCn9AQg0N0z6amkIbnIEth9-F2Xxv9tO8OVEZ-8UyfZ6gDj6J6_rYXqs01BOIWUN7i_azuw_nyP1_72SRRZP7ZLdZDxBlIw_HklRDosWUGUmPKwlz8Rv8UomdOifLlj0vo7UaCOC7kBS2c4m9gZ-3VUYZ6KN6nM7XfixUVhGGUeriFZU58gpaCEpP9Xsj5laN9jjlhGCbjMW5Q0ryhdSK5okXiFujrgzi9PySweRAWwEXaQ3L0E4BkxyRDJmks5MhxJuBGEXTIXOAjDkJ1sE51T9QHOEca3Bsjb4Qa7I2D6aPePC23VgC3GzOov3Eo_9ebV5wPNLTp6ue2ddIvIcywhK3s74L1RHeY6SX53NiY
The assessment of sleepiness in road traffic accidents and in the aviation industry caused by fatigue has shown that simulation tests are effective. One way of physically assessing sleepiness is by monitoring eye closure measures as this has been shown as an indicator of fatigue. The eyelid position may be a stable physiological measure of drowsiness/sleepiness.
Other physiological measurements such as Melatonin metabolite may provide opportunities to assess the Circadian process, but further research is necessary. (course)
Subjective Measurement of Sleep
Subjective measures of sleep include sleep questionnaires, sleep diaries/logs, rating scales on the estimated level of sleepiness and hardware devices. These measures can provide a broad overview of the level of hours of sleep, the consistency in sleep, sleep propensity and insomnia. They are also a quick and cost-effective wat to estimate sleepiness. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5511283/
Epworth Sleepiness Scale
https://link.springer.com/article/10.1007/s11325-020-02015-2
One of the most used measurements
Simple, widely used self-administered questionnaire
Measures levels of daytime sleepiness
Individuals are asked to rate their usual chance of dozing off in eight typical situations typically encountered in daily life (such as sitting and reading, watching tv, passenger in a car,etc)
Higher scores indicate a higher propensity to fall asleep
ESS is high in individuals with obstructive sleep apnea (OSA) and narcolepsy and low in individuals with insomnia
Validation studies indicates a significant correlation between ESS score and sleep latency using PSG
Karolinska Sleepiness Scale
Subjective rating scale that assesses sleepiness by using physiological and behavioural changes when sleepiness ratings are relatively high
Consists of a 9-point Likert scale
Self-reported, subjective assessment of an individual’s level of drowsiness at the time
Often used in research studies of shift work, sleep deprivation and driving
Good correlation with PSG measurements and performance based measures (worsening of performance is associated with increased KSS values)
Contact hardware devices to assess sleep
The prevalence of tracking sleep has increased in recent years. Predictive modelling based on heart rate assessment, respiratory rate and movement allows for certain devices to estimate levels of sleep and provide predictions in breakdowns in types of sleep. There are a variety of devices that can do this, such as the iWatch, ring devices and wearable tracker devices – a comparison of the performance and reliability of some od these hardwared devices can be found here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5971842/#ref-42
However, there is research indicating that fitness trackers and phone apps have the tendency to underestimate sleep disruptions and to overestimate total sleep times and sleep efficiency. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5971842/#ref-32
Although subjective measurements of sleep are useful to highlight the awareness of sleep deficit, we do need objective measures as well. Many healthcare professionals and clinical populations report an increased level of sleep restriction. This leads to many professionals accepting a new baseline of decreased alertness and performance as well as changes in mood. The detriments of this can be reversed, but it can also have long-term ramifications on physical, mental and emotional health. It is not possible to repay sleep debt and the impact of sleep restriction is constant and will most likely remain.
Conclusion
There are two separate, but interrelated processes involved in the governing of sleep – the homeostatic sleep drive and the circadian rhythm. These processes can be influenced by external factors such as physical, mental and physiological conditions, the level of lighting and even the work patterns in one’s profession. Both objective and subjective measurement of sleep have a role and is needed in making behavioural changes.
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