Circadian rhythm disorders

What it is

Circadian rhythm sleep-wake disorders (CRSWDs) are a group of conditions in which the body's internal 24-hour biological clock becomes misaligned with the external light-dark cycle or with social and work schedules, disrupting sleep, hormone secretion, metabolism, and organ function. Also called sleep-wake disorders, CRSWDs affect an estimated 3% of the general adult population, with delayed sleep-wake phase disorder (DSWPD) — the most common subtype — affecting up to 7–16% of adolescents. In Canada, roughly 1.5 million workers are employed on rotating or permanent night shifts, making shift work disorder one of the most clinically significant presentations seen in primary care.

The body's master clock sits in the suprachiasmatic nucleus (SCN) of the hypothalamus. It receives light signals through the retina to synchronize internal rhythms with the day-night cycle. When that synchronization breaks down — whether from shift work, jet lag, genetic clock variants, or chronic evening light exposure — the consequences extend well beyond tiredness. Cortisol, melatonin, insulin, reproductive hormones, and thyroid function all follow circadian patterns; misalignment disrupts each of them.

Causes and mechanism

CRSWDs arise when the internal clock is shifted relative to conventional clock time, or when external time cues (primarily light) conflict with the body's biological night. The main subtypes differ by cause and direction of shift:

Subtype	Clock direction	Typical population	Primary cause
Delayed sleep-wake phase disorder (DSWPD)	Runs late	Adolescents, young adults	Genetic + evening light exposure
Advanced sleep-wake phase disorder (ASPD)	Runs early	Older adults	Age-related clock changes
Shift work disorder	Forced misalignment	Night/rotating shift workers	Schedule conflicts with light-dark cycle
Jet lag disorder	Temporary misalignment	Travellers	Rapid transmeridian travel
Irregular sleep-wake rhythm disorder	Fragmented, no rhythm	Neurodegenerative disease, severe illness	Loss of SCN input or function

Several factors amplify misalignment regardless of subtype. Excessive artificial light at night — particularly blue-wavelength light from screens — suppresses melatonin and delays the clock. Irregular meal timing disrupts peripheral clocks in the liver and pancreas independently of the SCN. Chronotype (morning vs. evening preference) is partly heritable, with variants in clock genes such as PER3 and CRY1 identified in familial DSWPD.

Symptoms and diagnosis

Symptoms vary by subtype but commonly include difficulty falling asleep or waking at required times, excessive daytime sleepiness, non-restorative sleep, cognitive fog, irritability, and reduced physical performance.

From a hormonal standpoint, circadian misalignment produces measurable downstream effects:

• Cortisol: the normal morning cortisol awakening response is blunted; evening cortisol rises, contributing to insulin resistance and poor sleep.
• Melatonin: chronic evening light exposure suppresses nocturnal melatonin secretion, worsening sleep quality and delaying the clock further.
• Insulin and glucose: meal timing misaligned with the metabolic clock impairs pancreatic insulin secretion rhythms, raising postprandial glucose even in otherwise healthy individuals.
• Reproductive hormones: shift workers show altered LH pulsatility and changes in estrogen and progesterone patterns; menstrual irregularities are documented in women working rotating shifts.

Diagnosis typically involves:

1. Sleep diary — 2–4 weeks of self-reported sleep and wake times to identify consistent patterns.
2. Actigraphy — wrist-worn accelerometer tracking movement and light exposure over days to weeks; available through sleep clinics across Canada.
3. Dim light melatonin onset (DLMO) — measures the timing of melatonin secretion under dim light conditions, providing the most precise marker of circadian phase. Salivary DLMO testing is offered by select Canadian reference labs.
4. Hormone panel — diurnal cortisol (4-point salivary or serum), TSH, fasting glucose, and insulin may be ordered where metabolic or hormonal consequences are suspected. LifeLabs and Dynacare both process diurnal cortisol panels with a physician requisition.

Treatment options

Treatment targets clock resetting, light exposure correction, and management of any hormonal or metabolic consequences that have developed.

Light therapy is the most evidence-supported intervention for delayed circadian phases. Morning exposure to a 10,000-lux light box for 20–30 minutes advances the clock by suppressing residual melatonin earlier in the day. Timing relative to the individual's DLMO matters; a clinician familiar with circadian biology can guide this.

Melatonin works as a clock-shifting signal, not a sedative. Low doses (0.5–1 mg) taken 4–6 hours before the desired sleep time are more effective for phase-advancing the clock than the 5–10 mg doses common in Canadian pharmacies. Melatonin is available over the counter in Canada; Health Canada classifies it as a natural health product (NPN), and dosing guidance varies widely on packaging.

Chronotherapy involves progressively delaying or advancing sleep times over days to weeks to shift the clock to a target schedule. It requires careful planning and is best supervised.

Sleep hygiene and light avoidance — consistent wake times, reduced screen exposure in the 2–3 hours before bed, and morning outdoor light — reinforce clock stability and are appropriate first steps for mild presentations.

Shift work management combines strategic light therapy, timed melatonin, and carefully scheduled sleep opportunities to reduce the physiological burden of night work. Canadian occupational health guidelines recommend limiting consecutive night shifts and providing adequate recovery time between rotations.

Where significant metabolic or hormonal disruption has developed — insulin resistance, cortisol dysregulation, or reproductive hormone imbalances — referral to an endocrinologist or sleep medicine specialist is appropriate. Canadian patients can access sleep medicine through provincial referral pathways; telehealth platforms including Felix, Maple, and Cleo offer initial assessment for sleep concerns in most provinces.

When to see a clinician in Canada

Seek assessment if you consistently cannot fall asleep until very late and struggle severely to wake in the morning, to the point that work or school functioning is affected. Night-shift workers experiencing worsening fatigue, mood changes, digestive symptoms, or metabolic abnormalities over time should not attribute these solely to "getting used to it" — the hormonal consequences of long-term shift work are cumulative. Women with irregular menstrual cycles or fertility difficulties who also have significant sleep disruption should mention both to their physician, as the connection between circadian misalignment and reproductive hormone patterns is clinically relevant. Abnormal cortisol or fasting glucose results in someone with a history of night shift work or chronically irregular sleep warrant a conversation about circadian contributors.

Limitations and open questions

Research is still emerging on the optimal melatonin dose and timing protocol for each CRSWD subtype; most published trials use small samples and short follow-up periods. The long-term reversibility of metabolic and reproductive hormone changes in former shift workers is not well established. Health Canada has not issued specific clinical guidance on CRSWD management, and provincial pharmacare plans do not uniformly cover light therapy devices or actigraphy, creating access gaps for lower-income patients. The role of chrono-nutrition — aligning meal timing with the biological clock as a therapeutic tool — is promising in early trials but lacks the evidence base to support firm clinical recommendations. Individual variation in clock gene variants means that population-level timing protocols may not suit every patient, and personalized circadian medicine remains largely a research-stage concept.