Glucose, insulin and your circadian rhythm.
The purpose of the circadian rhythm is to optimize energy production and utilization by temporally separating opposing metabolic processes. It is a survival mechanism for energy conservation. The circadian clock modulates a number of metabolic processes including cholesterol synthesis, glucocorticoid secretion, hepatic gluconeogenesis and glucose tolerance. When the relationship between the endogenous and exogenous factors is disrupted a state known as "circadian misalignment" results and has been associated with the development of metabolic disease.
The natural pattern of glucose tolerance is that there is the highest tolerance in the morning with a a gradual decrease into the evening and lowest mid-sleep. Glucose patterns, in recent studies, have appeared to be heavily influenced by the circadian clock.
Melatonin and cortisol are the primary hormones connecting the rhythms of the clock to the peripheral organs. At the start of the light cycle, light is detected by the SCN which then signals the pineal gland to inhibit melatonin synthesis. During this time the sympathetic nervous system promotes the release of cortisol, when the retina detects the start of the dark cycle, the pineal gland and the parasympathetic nervous system are activated leading to the secretion of melatonin and the inhibition of cortisol. When melatonin binds to receptors, insulin secretion is inhibited. Thus resulting in low insulin levels during the night while melatonin levels are high.
Patients with Type 2 Diabetes have been found to have reduced melatonin levels and this is represented by the up-regulation of pancreatic melatonin receptors. While melatonin appears to have an antagonistic effect on he secretion of insulin, melatonin may influence the amplitude of insulin secretion more than dictate the actual oscillating patterns of insulin.
Daily behavioral patterns are what cause circadian misalignment in humans. Improper eating schedules, dark/light exposure (i.e artificial light at night), shift work, avoidance of circadian-consistent sleep time etc. Circadian misalignment caused by shift-work is a known-independent risk factor for type 2 diabetes and increased risk of cardiovascular disease.