Sleep and Circadian Rhythms in Illness
All physiology and behaviour shows a 24-hour rhythm. This is driven by an endogenous circadian network based upon a molecular feedback loop, termed the molecular clock. There is a molecular clock present in all cells of the body. These clocks need to function in synchrony with each other and with the environmental day. In mammals this is achieved via a master circadian pacemaker located in the suprachiasmatic nuclei (SCN) of the brain.
The SCN is aligned/entrained to the environmental light-dark cycle by specialised photoreceptors within the eye. In turn the SCN co-ordinates the activity of the entire circadian system. If this entrainment pathway is disrupted at any point, then clocks in different tissues can become uncoupled. This in turn results in a state of internal desynchrony of the 24-hour circadian network.
Our internal circadian network allows us to predict time, or at least predict regular events within the environment. Our bodies need the correct materials in the right place, in the right amount, at the right time of day, and the circadian system can anticipate these different needs. By anticipating the approaching day our bodies are prepared in advance so that the 'new' environment can be exploited immediately. Blood pressure and metabolic rate, along with many biological processes, rise before the new dawn. If we merely responded to the light of dawn to switch us from sleep into activity, then valuable time would be wasted getting our energy usage, senses, immune system, muscles and nervous system tuned-up for action. It takes several hours to switch from sleep to activity, and a poorly adapted biology would be a major disadvantage in the battle for survival.
It is often difficult to disentangle the impact of circadian rhythm vs sleep disruption, the two are inextricably linked. This led the SCNi team to develop the collective term Sleep and Circadian Rhythm Disruption or 'SCRD' as a shorthand for these phenomena. SCRD is a feature shared by some of the most challenging diseases of our time:
- mental illnesses such as schizophrenia, bipolar disorder and depression;
- neurological conditions like Alzheimer’s, stroke and multiple sclerosis;
- developmental disorders such as autism;
- serious disorders of the eye.
The emerging understanding of why SCRD is co-morbid with many serious illnesses and abnormal health states, and the realisation of the stark consequences of SCRD on health, provides an unprecedented opportunity to develop novel evidence-based and condition-specific approaches for the diagnosis and treatments of SCRD across the broad spectrum of health.