Phase angle corresponding to the maximal value of the rhythmic parameter studied.

Action spectrum:
Spectral sensitivity for an effect triggered by light. The action spectrum for the sensitivity of the eye is the V(lambda) curve. The c(lamda) function proposed by Prof. Gall corresponds aprox. to the action of spectrum for night-time melatonin suppression by light.

Advanced Sleep Phase Syndrome:
This is a condition in which a patient wakes up too early in the morning and gets sleepy early in the evening. It is common in older people.

The measure of one half of the extent of the rhythmic change estimated by the mathematical model (e.g., cosine curve) best fitting to the data (e.g., the difference between the maximum and the rhythm-adjusted mean of the best fitting curve).

Biological clock:
Self-sustained oscillators which generate biologic rhythms in absence of external periodic input (e. g., at the gene level in individual cells).

Biological rhythm:
A cyclical, repeated variation in a biological function.

Blue light hazard: 
Irreversible photochemical damage of photoreceptors in the eye by high radiances of short wave optical radiation (UV- ca. 500nm).

Brain waves:
Electrical signals sent in the brain. They can be detected by electrodes placed on the scalp. Examples of brain waves: alpha waves with a frequency of 8-13 cycles per second that are produced when a person is relaxed but awake; beta waves with a frequency of greater than 12 cycles per second that are produced when a person is awake and alert; theta waves with a frequency of 4-7 cycles per second that are produced during light sleep; delta waves with a high amplitude and a frequency of less than 4 cycles per second that are produced during deep sleep.

Bright-light therapy:
The use of bright lights (>1000 lux) for certain periods of time at a certain time of day to reset one’s biological clock.

Candela (Cd). The unit of luminous intensity 1 cd= 1 lm/sr. A candle has typically 1 cd, an incandescent lamp ca. 100 cd, the sun has ca. 3*10^27 cd.

The science of investigating and objectively quantifying phenomena and mechanisms of the biologic time structure, including the rhythmic manifestations of life. Term derived from: Chronos (time), bios (life), and logos (science).

An agent capable of influencing biologic rhythm parameters (e.g., the phase setting).

Changes in an individual’s biologic time structure preceding, coincident or following functional disorders or organic disease and/or time-dependent manifestation of disease.

Use of treatment timed according to the stages in the sensitivity-resistance cycles of target (or non target) tissues and organs (or of the organism as a whole) to enhance the desired pharmacologic effect and/or reduce undesirable side effects of drugs or other therapeutic agents.

Circa-rhythm with a naturally synchronized period of the lunar cycle.

Internal rhythms, free running in constant conditions with a period slightly deviating from the environmental cycle, by which they are synchronized in nature (in contrast to non circa-rhythms).

Circadian rhythm:
Circa-rhythm of metabolic, physiological or behavioral processes with a naturally synchronized period of 24 hours (the term ‘circadian’ is derived from the Latin circa meaning about and diem meaning day).

Biological rhythms with a naturally synchronized period of the prevailing tidal cycle (periods of 12.4h, 25.8h or otherwise).

A rhythm with a period of about 30 (±5) days. Includes, in mature women during the time of ovarian activity, the menstrual cycle. The term is preferred to the term “menstrual” because rhythms of this frequency are found in premenstrual girls, postmenopausal women and in men.

Color temperature:
Unit: Kelvin (K). Color of a light source, corresponding to the color of black body at that temperature. Color temperature is usually only given for lamps that are more or less “white”, that means with color coordinates close to the black body curve.

Events occurring twice a day, i.e. at dawn and dusk.

Timing mechanism, in which the parameter studied, is not involved in the causal loop (in contrast to hourglass).

Recurrence of events, without necessarily being of periodic nature (in contrast to rhythm).

Cortisol (hydrocortisone) is a steroid hormone synthesized and released by the adrenal cortex of the adrenal glands; it is important for normal carbohydrate metabolism and response to stress.

Crepuscular activity:
An activity mainly performed at dusk and/or dawn.

Critical photoperiod:
That 24h LD ratio at which 50% of the population under study is photoperiodically switched from one state to another, e.g. into flowering from non-flowering, or into development from diapause.

Dawn simulation: 
Mimicking of the slow increase in natural light before and during sunrise with artificial lighting.

Deep sleep:
Stages 3 and 4 of NREM sleep during which delta waves are produced.

Delayed Sleep Phase Syndrome:
This is a condition in which a patient doesn’t get sleepy until early in the morning the next day and sleeps late into the morning. It is common in older teens.

State of two or more previously synchronized rhythmic variables that have ceased to exhibit the same frequency and/or the same acrophase relationships and show different than usual and/or changing time relations.

A period of arrested growth or reduced physiological activity, commonly induced by a seasonal change in photoperiod (i.e. day-length); a term used mainly for invertebrates, especially insects.

Activity or event occurring in the day between dawn and dusk.

A state of non activity during which sleep brain waves cannot be detected.

Electroencephalogram (EEG):
A measurement of brain wave activity. This data is collected through sensors on the scalp.

Electromyogram (EMG):
A measurement of muscle tension. The data is collected through sensors.

Electrooculogram (EOG):
A measurement of eye movement. This data is collected through sensors.

Endogenous rhythm:
An oscillating system capable of self sustained oscillations.

Coupling of an endogenous rhythm to an environmental oscillator with the result that both oscillations have the same frequency. In contrast to masking, the phase of the endogenous rhythm is affected by entrainment.

Exogenous rhythm:
An oscillating system not capable of self sustained oscillations, but passively driven by external factors.

External desynchronization:
Desynchronization of a biologic rhythm from an environmental cycle.

Frequency manifestation of self sustained oscillations, with a periodicity deviating from (eventual) Zeitgebers.

Frequency (f):
The number of cycles occurring per time unit; f is the reciprocal of the period (t).

Frequency ranges:
Groups of frequencies (or periods) frequently encountered in biologic rhythms. (Circadian frequency range: rhythm with periods of about one day, i. e., by definition > 20 to < 28 h).

Winter or cold season dormancy.

Timing mechanism, in which the parameter studied is part of the causal loop (in contrast to clock).

A small brain structure at the base of the diencephalon. The structure is involved in functions including homeostasis, emotion, thirst, hunger, circadian rhythms, and control of the autonomic nervous system.

Infradian rhythm:
Non circa-rhythm with a period longer than 24 hours.

Internal desynchronization:
State in which two or more previously synchronized variables within the same organism have ceased to exhibit the same frequency and/or the same acrophase relationships and show different than usual and/or changing time relations.

Jet lag:
Desynchronization and its clinical effect after rapid movement over several time zones (after movements over time zones, e.g. transmeridian flights).

Lighting regime:
The light-dark cycle (LD; i.e. 12h light and 12h darkness), or constant light (LL), or constant dark (DD) conditions used for chronobiologic studies.

Longitudinal sampling:
Study of the same subject or of a group of subjects over numerous cycles.

Masking effect:
Overruling of the expression of endogenous rhythm by random or nonrandom environmental stimuli only temporarily, without affecting the phase of that rhythm (in contrast to entrainment).

A hormone produced rhythmically in vertebrates by the pineal gland, a pea sized organ at the center of the human brain.

Activity or event occurring in the night between dusk and dawn.

Non circa-rhythms:
Endogenous rhythms with periods that have no counterpart in the natural environment, but are characterized by allometric relationships (in contrast to circa-rhythms).

Non-Rapid Eye Movement Sleep (NREM):
A type of sleep that is divided into four stages. Usually about 75% of the night is spent in NREM Sleep.

Activity or event occurring on a 24h basis (i.e. daily).

Mechanism leading to the manifestation of a rhythmic phenomenon (oscillation), that either is self sustained (pacemaker), or depends on another oscillator (passive or slave-oscillator), or is characterized by a decrease in amplitude (damped oscillator).

A functional entity capable of self-sustaining oscillations.

The highest point in a series of measurements obtained as a function of time.

Time after which a defined phase of the oscillation re-occurs.

Instantaneous state of an oscillation within a period (reference point).

Phase advance:
Shortening of period for one to a few cycles (denoted by a plus sign).

Phase angle:
Position of a parameter value relative to the period, expressed in fraction of the period or in time, if the length of period is known.

Phase angle difference:
Difference in position between two phase-angles in two coupled oscillations.

Phase delay:
Lengthening of period for one to a few cycles (denoted by a minus sign).

Phase reference:
Time point chosen by the investigator as reference for the estimation of the timing of a rhythm.

Phase-response curve (PRC):
The 24h profile of an organism’s phase shifts in response to environmental signals.

Phase shift:
Displacement of an oscillation along the time axis (see phase advance and phase delay).

In a light-dark regimen the duration of the light span.

The seasonal day-length responses that cause altered physiological states such as flowering or non-flowering for example.

Rapid Eye Movement Sleep (REM):
A type of sleep during which the body is paralyzed and the eyes move. Dreaming occurs during this sleep.

Range of entrainment:
Range of frequencies within which a self sustained oscillation can be entrained by a Zeitgeber.

Temporal insensitivity to a Zeitgeber.

Periodic recurrence of events; not synonymous with cycle.

In a light-dark regimen the duration of the dark span.

A neurotransmitter that affects how alert one is.

Shift work:
Transient or permanent change in work schedule in relation to the social surroundings.

Slow-wave sleep:
Stages 3 and 4 of NREM sleep, during which the brain waves are large and slow. These are usually the deeper stages of sleep.

Suprachiasmatic nucleus (SCN):
Group of neurons situated above the optic chiasm in the vertebrate hypothalamus exhibiting an endogenous circadian oscillation acting as circadian pacemaker, receiving external phase information via the retina.

State in which two or more oscillations have the same frequency due to mutual or unilateral influences (referring both to entrainment and to masking).

Transverse sampling:
Sampling of a group of subjects over one cycle of a rhythm.

The ‘natural’ period of a biological rhythm free-running in constant conditions.

Temperature coefficient (Q10):
It is defined as the change in the rate of a process as a result of increasing the temperature by 10°C. For most physiological reactions the value is close to 2 as increasing the temperature has a large effect on the reaction rate. For biological clocks the value is about 1 because they are temperature compensated.

Temperature compensation:
The phenomenon exhibited by biological clocks that results in them not responding to different ambient temperatures in a way that would be expected according to normal physiological Q10 principles. In simple terms Biological Clocks are not effected by different temperatures, whereas other physiological systems are.

Temperature cycle.

The lowest point in a series of measurements obtained as a function of time.

Ultradian rhythm:
Non circa-rhythm with a period less than 24 hours.

From the German for meaning “time giver”. A periodic environmental signal that entrains some biological rhythm, for example light, temperature, or social Zeitgebers. It is important to understand that the “Zeitgeber” does not induce a rhythm but determines its arrangement in time.