Blood pressure is a measurement of the force exerted by circulating blood against the walls of the body’s arteries, the major blood vessels. This measurement is expressed as two numbers: the systolic pressure, which is the higher number reflecting the pressure when the heart beats, and the diastolic pressure, the lower number representing the pressure between heartbeats when the heart rests. A reading is considered low blood pressure, or hypotension, when it falls below 90 millimeters of mercury (mm Hg) systolic and 60 mm Hg diastolic. While blood pressure naturally adjusts throughout the day in response to activity and stress, it also follows a predictable pattern related to the sleep-wake cycle, which includes a normal drop at night. This physiological fluctuation sets the context for understanding whether a drop in pressure during sleep can become dangerous.
The Immediate Risk of Nocturnal Hypotension
For most healthy people, the natural decrease in blood pressure that occurs while sleeping is entirely safe and does not pose a risk of death. The danger arises only when this nighttime drop is pathologically severe, causing the blood pressure to fall so low that it leads to insufficient blood flow, or ischemia, to essential organs.
When blood pressure drops excessively, the force needed to push blood through the smallest vessels is lost, leading to a lack of oxygen supply to tissues. This severe, sustained drop can precipitate a life-threatening event, primarily by causing a myocardial infarction, or heart attack, or an ischemic stroke in the brain. The heart muscle itself, especially if already compromised by coronary artery disease, may become severely ischemic during non-REM sleep when blood pressure is at its lowest point.
Severe nocturnal hypotension is particularly risky for individuals with pre-existing conditions like advanced vascular disease, where blood vessels may already be narrowed and unable to compensate for the reduced pressure. If the blood pressure falls below the body’s ability to maintain perfusion—the lower limit of autoregulation—tissue damage occurs, which is the mechanism by which it can be fatal.
How Blood Pressure Naturally Changes During Sleep
The body’s blood pressure follows a clear 24-hour rhythm, known as the circadian rhythm, which is tightly linked to the sleep-wake cycle. This rhythm is regulated by the autonomic nervous system, which automatically controls functions like heart rate and vascular tone. During the transition from wakefulness to sleep, the body shifts from a sympathetic dominance—the “fight or flight” state—to a parasympathetic dominance, which promotes “rest and digest” functions.
This change in autonomic activity causes a reduction in heart rate and a decrease in the resistance of blood vessels, leading to a predictable lowering of blood pressure. For a person with normal blood pressure, this drop amounts to a 10% to 20% reduction from daytime levels. This expected decline is known as “dipping” and is a healthy physiological response to rest. The lowest blood pressure readings usually occur during the deeper stages of non-REM sleep.
Medical Causes of Dangerous Blood Pressure Drops
The conditions that lead to dangerously low nocturnal blood pressure are rooted in an underlying medical pathology that exaggerates the normal dipping response. One of the most common causes is the side effect of medications, particularly those prescribed to manage high blood pressure (hypertension). If anti-hypertensive drugs, such as alpha-blockers or diuretics, are dosed incorrectly or taken too close to bedtime, their effect can peak during the night, pushing an already dipping blood pressure to dangerously low levels. This over-treatment can be pronounced in older adults.
Another significant category of risk involves conditions that cause autonomic dysfunction, where the nervous system’s ability to regulate blood pressure is impaired. Conditions like Parkinson’s disease, multiple system atrophy, and diabetes-related neuropathy can damage the nerves responsible for controlling vascular constriction and heart rate. This damage prevents the body from making the necessary adjustments to stabilize blood pressure when lying down or when the pressure naturally dips during sleep, leading to a profound, unregulated drop.
Severe systemic illnesses also significantly increase the risk of nocturnal hypotension. Conditions that reduce the overall volume of circulating blood, such as severe dehydration, major blood loss, or advanced heart failure, lower the baseline blood pressure. When an individual with an already low baseline pressure goes to sleep, the normal nocturnal drop can easily cross the threshold for organ perfusion failure. Furthermore, severe infections like sepsis cause massive dilation of blood vessels, which dramatically lowers resistance and can lead to life-threatening shock, even during rest.
Recognizing Symptoms and Monitoring Low Blood Pressure
Since the most severe drops happen during sleep, the immediate warning signs often manifest upon waking. Orthostatic hypotension is a significant drop in blood pressure when moving from a lying or sitting position to standing. Symptoms include dizziness, lightheadedness, or feeling faint immediately after getting out of bed. This suggests that blood pressure was critically low while recumbent and the body could not compensate quickly enough upon standing.
Other symptoms that may indicate severe nocturnal hypotension include waking up feeling confused, disoriented, or experiencing persistent weakness and fatigue. A person may also experience blurred vision. To accurately diagnose the issue, a physician may recommend 24-hour ambulatory blood pressure monitoring, which takes readings frequently throughout the night while the person sleeps. Any reading consistently below 90/60 mm Hg, especially if accompanied by symptoms, warrants a discussion with a healthcare provider to investigate the underlying cause and adjust medications if needed.