Advanced Sleep Phase Disorder (ASPD)

Is a disorder in which the timing of sleep and the peak period of alertness are advanced several hours realtive to our societal clock.  People who suffer from ASPD have an urge to go to sleep much earlier than what is considered a ‘normal’ time, which results in an early wake time.

Blue Light

When referring to circadian entrainment, blue light is light in the 480-490nm range.  This is what our ipRGCs respond to and signal to our body clock that it is daytime.  We need blue light during the day, however want to avoid it at night.


A chronotype is the behavioral manifestation of underlying circadian rhythms of myriad physical processes. A person's chronotype is the propensity for the individual to sleep at a particular time during a 24-hour period


Meaning "about a day". The term circadian comes from the Latin circa, meaning "around" (or "approximately"), and diēm, meaning "day".


A steroid hormone, in the glucocorticoid class of hormones, is made in the adrenal glands.  Cortisol can help control blood sugar levels, regulate metabolism, help reduce inflammation, and assist with memory formulation.

Cortisol Awakening Response (CAR)

The cortisol awakening response (CAR) is the change in cortisol concentration that occurs in the first hour after waking from sleep.  It is associated with feeling alert and awake.

Delayed Sleep Phase Disorder (DSPD)

A disorder in which a person’s sleep is delayed by two hours or more beyond what is considered as their ‘true’ biological bed time.  People who suffer from DSPD can experience a shortened sleep cycle if their wake up is determined by their social clock (work or school etc.)

Endogenous Circadian Rhythm

Also referred to as our "free running" body clock is the inbuilt wake/sleep pattern we exhibit in the absence of entrainment cues.  This period is close to but not exactly 24 hours.


The synchronising of our endogenous rhythm with the 24 hour day. The entrainment cue "time giver" for this process is the light/dark pattern of our day.

Homeostatic Process

The increasing drive to sleep as a result of the gradual build up of adenosine in the body from the moment we wake.  This forms one part of the two-process model of sleep.


A small region of the brain that has a vital role in controlling many bodily functions including the release of hormones from the pituitary gland.

Intrinsically Photosensitive Retinal Ganglion Cells (ipRGCs)

A melanopsin expressing photoreceptor located in the retina of the mammalian eye.  This is a non-image forming photoreceptor that is sensitive to light in the 480-490nm range and has a direct path to the SCN.


A type of photopigment belonging to a larger family of light sensitive retinal proteins called opsin.  Melanopsin is expressed by the ipRGCs in response to light.


hormone primarily released by the pineal gland that regulates our sleep/wake cycle.

Phase Shift

A change (either an advance or delay) in the phasing of an organisms endogenous circadian rhythm in response to an acute stimulus (light).  Jet lag is an example phase shifting.


The quantum of electromagnetic radiation, or more simply a particle of light.

Rods and Cones

Image forming photoreceptors located in the mammalian retina.  Rods are responsible for vision at low light levels (scotopic) whereas Cones are active at higher light levels (photopic) and are capable of colour vision and high spatial acuity.

Social Jetlag

The time difference between the regimented wake time of an individual on weekdays and their natural wake time on the weekends.  This discrepancy highlights a potential misalignment between and individuals endogenous body clock and their environment.

Spectral Power Distribution (SPD)

A graph showing the concentration of energy as a function of of wavelength across the visual spectrum.  This allows us to compare various light sources at a spectral level.

Suprachiasmatic Nucleus (SCN)

The master circadian pacemaker in mammals located in the hypothalamus and directly linked to the ipRGCs in the eye.


The "time giver" or "synchroniser" that anchors our endogenous clock to our environment.  For mammals this is light.