Feeling chilly or shivering immediately after your alarm goes off is a common experience. This sensation is not a sign of a faulty thermostat, but rather a direct result of your body performing a complex biological maneuver to prepare for sleep and then for waking. It reflects the temporary misalignment between your internal body temperature and the sudden need for active energy expenditure.
The Body’s Thermostat During Sleep
The perception of cold upon waking stems from the fact that your body actively lowers its core temperature overnight to promote and maintain sleep. To shed heat, the body employs vasodilation, widening the blood vessels, particularly in the extremities like the hands and feet. This mechanism effectively turns the skin into a radiator, allowing warmth to dissipate into the surrounding environment.
This heat loss is paired with a reduction in the body’s metabolic rate, which decreases the total amount of heat being generated internally. During non-rapid eye movement (NREM) sleep, the body’s thermoregulation is less precise than when you are awake. The result is a drop in core temperature by approximately 1 degree Celsius (1.8 degrees Fahrenheit) over the course of the night. This lower temperature set point is the physiological baseline from which you wake, making the surrounding air feel much colder by comparison.
The Circadian Timing Mechanism
The timing of this internal cooling and subsequent reheating is dictated by the body’s master biological clock, the circadian rhythm. This rhythm is governed by the suprachiasmatic nucleus (SCN), which coordinates nearly all 24-hour cycles, including body temperature. The SCN is programmed to cycle the core temperature downward in the evening as a signal to prepare for rest.
This internal clock ensures that the body’s temperature reaches its lowest point in the early morning hours, typically one to two hours before your habitual wake time. This timing reduces metabolic demand during the sleep period’s end. Although the warming process is already underway, the temperature remains at its lowest set point when you first open your eyes.
The Hormonal Wake-Up Call
The sudden transition from the body’s low-temperature, low-energy sleep state to active wakefulness is orchestrated by a rush of specific hormones. Just before and immediately upon waking, the body initiates the “cortisol awakening response,” a sharp surge in the stress hormone cortisol. This provides a metabolic boost necessary for arousal.
This hormonal signal works in conjunction with the nervous system to increase sympathetic activity, which involves the release of adrenaline (epinephrine). Adrenaline and cortisol both act to elevate the heart rate and blood pressure, simultaneously increasing the rate of cellular metabolism. This rapid increase in metabolic activity signals the body to begin producing heat again. The sudden shift from low resting metabolism to an activated, heat-generating state registers as the cold feeling, sometimes triggering minor shivering as the body attempts to achieve thermogenesis.
Environmental Factors and Simple Solutions
While internal biology is the primary driver of the morning chill, external conditions can intensify the feeling. A common environmental factor is a bedroom temperature that is too low. Drafts from windows or doors can also create localized cold pockets that exacerbate the natural temperature drop.
Simple adjustments can help mitigate the discomfort of this physiological transition. Using layered bedding allows you to trap heat more effectively and adjust to the body’s changing temperature needs throughout the night. Setting a smart thermostat to increase the room temperature 30 minutes before your alarm can smooth the wake-up process. Having a thick robe and slippers immediately accessible encourages rapid containment of body heat and helps prevent the shock of contact with a cold floor.