The bottom number in your blood pressure reading, called diastolic pressure, measures the force on your artery walls between heartbeats. A high bottom number (80 mmHg or above, according to American Heart Association guidelines) typically results from increased resistance in your smaller blood vessels, meaning they’re tighter or stiffer than they should be. Several factors can drive this, from everyday habits to underlying medical conditions.
What the Bottom Number Actually Measures
Your blood pressure reading has two numbers. The top number (systolic) captures pressure when your heart contracts. The bottom number (diastolic) captures pressure when your heart relaxes between beats. A high diastolic number means your blood vessels aren’t relaxing enough during that rest period, keeping pressure elevated even when the heart isn’t actively pumping.
This matters because your heart spends more time in that resting phase than in active contraction. If the pressure stays high during rest, your heart and blood vessels are under constant strain. In one study of older adults, those with isolated diastolic hypertension (high bottom number, normal top number) developed heart failure at nearly five times the rate of those with normal readings.
Who Gets a High Bottom Number
Isolated diastolic hypertension, where only the bottom number is elevated, typically affects people under 55. This pattern is distinct from what happens as you age. Over time, arteries stiffen, which pushes the top number up while the bottom number naturally drops. So a high diastolic reading in a younger person points to a different process: excess tightening of the smaller blood vessels rather than large-artery stiffness.
Three of the biggest risk factors identified by researchers are carrying excess weight, having sleep apnea, and smoking. Beyond that, experts don’t fully understand why some people develop a high bottom number while their top number stays normal.
How Blood Vessel Tightening Works
Two main biological pathways keep blood vessels constricted and drive diastolic pressure up. The first involves sodium retention. When your body holds onto too much sodium, it changes how calcium moves in and out of the smooth muscle cells lining your blood vessels. Those muscles contract more than they should, narrowing the vessels. People with this pattern tend to see improvement when they cut sodium intake significantly.
The second pathway involves the renin-angiotensin system, a hormone cascade your kidneys use to regulate blood pressure. When this system is overactive, it triggers vessel constriction through a different mechanism that also involves excess calcium inside muscle cells. In many people, both pathways operate at the same time, which is one reason high diastolic pressure can be stubborn to control.
Sleep Apnea’s Outsized Role
Obstructive sleep apnea is one of the most common and underrecognized drivers of a high bottom number. When your airway collapses repeatedly during sleep, your oxygen levels drop and your sleep fragments. Your nervous system responds by flooding the body with stress signals, keeping blood vessels constricted throughout the night and into the day.
This isn’t just a momentary spike. The repeated oxygen drops activate a hormonal chain reaction: your body ramps up a blood-pressure-raising hormone system (the same renin-angiotensin system mentioned above), increases levels of a vessel-constricting compound, and reduces levels of nitric oxide, the molecule that normally helps vessels relax. Over time, this remodels blood vessels to stay tighter at baseline. If your bottom number is persistently high and you snore, wake up tired, or have been told you stop breathing in your sleep, sleep apnea may be driving the problem.
Everyday Habits That Raise the Bottom Number
Alcohol has a measurable, dose-dependent effect on diastolic pressure. A large Japanese longitudinal study found that even light drinking (half a drink to one drink per day) was associated with higher blood pressure in women. When people who drank one to two drinks daily stopped, their diastolic pressure dropped by about 1.1 mmHg in women and 1.6 mmHg in men. That may sound small, but across years, even modest sustained reductions lower cardiovascular risk.
Excess sodium causes your body to retain fluid, increasing the volume of blood your vessels have to handle and activating the constriction pathway described earlier. High stress levels keep your sympathetic nervous system in overdrive, which directly tightens blood vessels. Physical inactivity allows those vessels to lose their flexibility over time. Smoking damages the inner lining of blood vessels, reducing their ability to dilate properly. Each of these factors can independently nudge diastolic pressure up, and they often stack.
Medications That Can Raise It
Several common medications raise blood pressure as a side effect, and if your bottom number crept up around the time you started a new medication, that’s worth investigating.
- Pain relievers (NSAIDs): Ibuprofen, naproxen, and similar anti-inflammatory drugs cause your body to retain water, which raises pressure. Regular use is a frequent culprit.
- Decongestants: Pseudoephedrine and phenylephrine, found in many cold and allergy products, narrow blood vessels to reduce swelling in your sinuses. That same narrowing raises blood pressure.
- Hormonal birth control: Pills and patches containing hormones can raise blood pressure in some people, particularly those over 35, those who smoke, or those carrying extra weight.
- Some antidepressants: Certain classes of antidepressants alter how the body processes brain chemicals in ways that can increase blood pressure.
- Stimulants: ADHD medications can speed up the heart rate and raise blood pressure.
- Caffeine: If you don’t consume it regularly, caffeine can cause short-term blood pressure spikes.
Less Common Medical Causes
Sometimes a high bottom number signals an underlying condition that’s actively pushing pressure up. Primary aldosteronism, where the adrenal glands produce too much of a hormone called aldosterone, is now considered the most common endocrine cause of hard-to-treat high blood pressure. Excess aldosterone tells your kidneys to hold onto sodium and water, directly increasing blood volume and vessel constriction.
Kidney problems, including narrowing of the arteries that supply the kidneys, can also drive diastolic pressure up. When the kidneys sense reduced blood flow, they activate the renin-angiotensin system aggressively, constricting vessels throughout the body. Other rarer endocrine conditions, like tumors of the adrenal gland or Cushing’s disease, can have similar effects, though these are uncommon.
If your bottom number stays elevated despite lifestyle changes and is resistant to treatment, these secondary causes are worth ruling out. They’re often identifiable through bloodwork and imaging, and treating the underlying condition can resolve the blood pressure problem entirely.
Why the Bottom Number Matters on Its Own
For years, most attention focused on the top number. But research has made clear that a high diastolic reading carries real risk, even when systolic pressure looks fine. In the Cardiovascular Health Study, participants with isolated diastolic hypertension had a heart failure rate of 19%, compared to 7% among those with normal pressure. After adjusting for other risk factors, the hazard ratio was 4.65, meaning the risk was nearly five times higher. There was also a strong trend toward higher cardiovascular mortality, though it didn’t quite reach statistical significance in that study.
The bottom line: a persistently elevated bottom number isn’t something to brush off just because the top number is normal. It reflects ongoing vascular resistance that stresses your heart and blood vessels over time, and it responds to the same interventions that help blood pressure overall: managing weight, reducing sodium and alcohol, staying active, treating sleep apnea if present, and reviewing your medications for anything that might be contributing.