Comprehensive Guide to Sleep, Consciousness, and Circadian Rhythms in Psychology

Psychological construct

A agreed-upon description and understanding of psychological phenomena that cannot be overtly measured or observed.

Consciousness

The level of awareness an individual has of their thoughts, feelings, perceptions and existence at any given moment.

Normal waking consciousness (NWC)

A state of consciousness in which an individual is awake and aware of internal and external stimuli.

Altered state of consciousness (ASC)

A state of consciousness that is distinctly different from normal waking consciousness in terms of the quality of experience and levels of awareness.

Sleep

A regular and naturally occurring altered state of consciousness involving a loss of awareness and disengagement from internal and external stimuli.

Sleep episode

A full duration of time spent asleep, made up of multiple repeated sleep cycles.

Sleep cycle

An approximately 90-minute period that repeats during a sleep episode, in which an individual progresses through stages of REM and NREM sleep.

REM sleep

A type of sleep characterised by rapid eye movement, high levels of brain activity and low levels of physical activity.

NREM sleep

Non-rapid eye movement sleep, characterised by a lack of rapid eye movement and divided into three stages: NREM 1, NREM 2 and NREM 3.

NREM stage 1

A light sleep transition stage where awareness of surroundings fades and the individual is still relatively easy to wake.

NREM stage 2

A light sleep stage in which the individual is fully asleep and spends most of their sleep time, although they are still relatively easy to wake.

NREM stage 3

A deep sleep stage where the individual is difficult to wake and may feel confused or drowsy if woken; sleepwalking and sleep talking are most likely to occur.

Circadian rhythm

A biological and behavioural process that occurs as part of a cycle lasting approximately 24 hours.

Sleep-wake cycle

A circadian rhythm consisting of the approximately 24-hour cycle of sleeping and being awake and alert.

Ultradian rhythm

A biological and behavioural process that occurs in a cycle lasting less than 24 hours.

EEG (electroencephalograph)

A device that detects, amplifies and records the electrical activity of the brain, measuring brain waves in terms of frequency and amplitude.

EMG (electromyograph)

A device that detects, amplifies and records the electrical activity of muscles.

EOG (electro-oculograph)

A device that detects, amplifies and records the electrical activity associated with eye movements.

Sleep diary

A self-reported record of an individual's sleeping and waking periods, including estimated sleep duration and judgements about sleep quality and nature.

Video monitoring

The use of camera and audio technologies to record and monitor an individual while they sleep.

Objective measurement

A measurement based on observable or measurable physiological responses that does not rely on the individual's personal judgement or report.

Subjective measurement

A measurement based on an individual's personal experiences, perceptions or reports.

Hypnogram

A graph used in sleep medicine to visualise the stages of sleep throughout a sleep episode.

Suprachiasmatic nucleus (SCN)

A pair of structures in the hypothalamus responsible for regulating an individual's sleep-wake patterns and circadian rhythm.

Melatonin

A hormone released by the pineal gland, typically at night, that promotes calmness and relaxation and makes it easier to fall asleep.

Pineal gland

A gland in the brain responsible for producing and releasing melatonin.

Cortisol

A hormone released by the adrenal cortex that increases alertness and arousal; cortisol levels typically rise in the morning.

Zeitgeber

An external environmental cue that influences and helps regulate an individual's circadian rhythm.

External cue

Information from the environment, such as light, that influences the circadian rhythm.

Internal cue

Information originating within the body, such as the expression and suppression of clock genes, that influences the circadian rhythm.

Sleep onset

The point at which an individual falls asleep.

Sleep deprivation

A state caused by inadequate quantity or quality of sleep, either voluntarily or involuntarily.

Partial sleep deprivation

A state in which an individual obtains some sleep within a 24-hour period, but the duration is too short or the quality of sleep is poor.

Full sleep deprivation

A state in which an individual obtains no sleep within a 24-hour period.

Sleep quantity

The amount of sleep an individual obtains.

Sleep quality

How well an individual sleeps.

Affective functioning

Changes in emotions and emotional responses that arise from sleep deprivation.

Behavioural functioning

Changes in actions and the ability to control behaviour that arise from sleep deprivation.

Cognitive functioning

Changes in mental processes that arise from sleep deprivation.

Sleep inertia

Performance impairment that occurs immediately after awakening, which can include slower reaction time, difficulty maintaining concentration and reduced awareness of the environment.

Microsleep

A brief, involuntary period of sleep, typically lasting around 1-10 seconds, during which awareness of external stimuli may decrease and the individual may have a fixed gaze or blank expression and little or no recollection afterwards.

Circadian rhythm sleep disorder

A sleep disorder that interferes with the typical regulation of the circadian rhythm of sleep, resulting in a disrupted sleep-wake cycle.

Delayed Sleep Phase Syndrome (DSPS)

A circadian rhythm sleep disorder in which sleep onset and waking occur later than desired or usual.

Advanced Sleep Phase Disorder (ASPD)

A circadian rhythm sleep disorder in which sleep onset and waking occur earlier than desired or usual.

Bright Light Therapy

A method of adjusting an individual's circadian rhythm through timed exposure to a high-intensity light source.

Sleep hygiene

The practices and habits that promote healthy sleep patterns.

Adaptation to zeitgebers

The process of adjusting to external environmental cues that influence the circadian rhythm.

Blue light

A type of light emitted naturally by daylight and artificially by devices that can act as a zeitgeber and promote wakefulness.

Sleep-wake shift

The biological shift during adolescence in which the circadian rhythm becomes delayed, causing sleepiness and sleep onset to occur later.

Neonate

A newborn infant, approximately 1-15 days old.

Infant

A child approximately 3-24 months old.

Child

A person approximately 3-12 years old.

Adolescent

A person approximately 12-18 years old.

Adult

A person approximately 18-60 years old.

Late adulthood

The life stage beginning at approximately 60 years of age.

Mental wellbeing

An individual's psychological state, involving their ability to think, process information and regulate emotions.

Sleep hygiene

The practices and habits that promote healthy sleep patterns.

Sleep hygiene — time

Sleeping and waking at consistent times that allow an individual to obtain an adequate amount of sleep.

Sleep hygiene — sound

Sleeping in a relatively quiet environment to minimise disturbances.

Sleep hygiene — light

Sleeping in a dark environment and reducing exposure to bright light close to bedtime.

Sleep hygiene — comfort

Sleeping in a comfortable environment that supports sleep.

Sleep hygiene — technology/devices

Avoiding bright screen use close to bedtime to reduce stimulation and exposure to light that may delay sleep.

Sleep hygiene — association with bed

Using the bed primarily for sleeping rather than activities such as studying, watching TV or using devices.

Sleep hygiene — food and drink

Avoiding large meals, caffeine and alcohol close to bedtime.

Sleep hygiene — exercise

Exercising earlier in the day and avoiding exercise close to bedtime.

Zeitgeber

An external environmental cue that influences and helps regulate an individual's circadian rhythm.

Adaptation to zeitgebers

The process of adjusting to external environmental cues that influence the circadian rhythm.

Examples of zeitgebers

Light, temperature, eating and drinking patterns, and exercise.

Daylight

The natural light an individual is typically exposed to during the day, which acts as a zeitgeber and promotes wakefulness.

Blue light

A type of light that can be emitted naturally or artificially and can act as a zeitgeber to influence the circadian rhythm.

Daylight as a zeitgeber

Daylight signals the SCN that it is daytime, promoting wakefulness and suppressing melatonin production.

Blue light as a zeitgeber

Blue light can act as an external cue that promotes wakefulness; exposure to blue light at night can delay sleepiness by suppressing or delaying melatonin production.

Artificial blue light

Blue light emitted by artificial sources such as phones, tablets, computers and LED lights that can act as a zeitgeber.

Temperature as a zeitgeber

External temperature can influence the quantity and quality of sleep; a cooler sleeping environment may support better sleep.

Ideal room temperature for sleep

Approximately 18.3°C has been suggested as an ideal room temperature for sleep.

Eating and drinking patterns as a zeitgeber

The timing, type and amount of food and drink consumed can act as external cues that influence the circadian rhythm.

Exercise as a zeitgeber

The timing of physical activity can act as an external cue that influences the circadian rhythm.

Light and the SCN

Light is detected by the eyes and provides information to the SCN about whether it is day or night, helping regulate the sleep-wake cycle.

Light and melatonin

Light exposure, particularly blue light, can signal the SCN to suppress or delay melatonin production, promoting wakefulness.

Darkness and melatonin

The onset of darkness signals the body that it is nighttime, allowing melatonin levels to rise and promoting sleepiness.

Sleep hygiene and circadian rhythm

Consistent sleep times, appropriate light exposure and healthy behaviours can support the regulation of the circadian rhythm and improve sleep.