Blood pressure changes throughout the day. During sleep, it normally drops about 10 to 20 percent below daytime levels. Then, in the hours before you wake up, it starts to rise. This morning surge happens during the same part of the day when heart attacks and strokes are most common.

Blood pressure usually stays higher through the morning and much of the workday, then eases in the evening and falls again during sleep. In many people with high blood pressure, especially older adults, that nighttime dip becomes smaller or disappears. This is one reason a doctor may recommend a 24 hour blood pressure monitor.

What does this mean for your home readings? A reading at 7 a.m. is not really comparable with one at 3 p.m. To see a meaningful pattern, measure at about the same times each day. Guidelines generally recommend checking in the morning before breakfast, coffee, and blood pressure medicine, then checking again in the evening.

Sleep matters too. In one study of 36 adults with untreated high blood pressure, just one night of shortened sleep raised blood pressure and heart rate over the following 24 hours, including the next morning.

Coffee can change the number more than you might expect.

In studies of people with high blood pressure, 200 to 300 milligrams of caffeine, roughly the amount in two or three cups of brewed coffee, raised systolic pressure by about 8 points and diastolic pressure by about 6 within the first hour. The effect can last three hours or longer. Across studies, the increase in systolic pressure has ranged from about 3 to 14 points.

Regular coffee drinkers often develop some tolerance, but not everyone does. In one trial, half of the participants still had their full blood pressure response to caffeine after five days of consuming 600 milligrams a day.

Age and high blood pressure may also make a difference. In a study of adults with an average age of 72, those with hypertension who drank five cups of instant coffee a day had 24 hour blood pressure readings about 5 over 3 points higher after two weeks than those who avoided coffee. People with normal blood pressure did not show the same difference.

Small details in your position can make a surprisingly big difference.

In a 2024 Johns Hopkins study of 133 adults, resting the arm in the lap raised systolic pressure by 3.9 points compared with supporting it on a desk at heart level. Letting the arm hang at the side raised it by 6.5 points.

Crossing your legs can also raise the reading. Studies have found increases ranging from about 2.5 to nearly 15 points. Sitting without back support raised the bottom number by about 6.5 points in one study. Keeping the arm below heart level has added anywhere from 4 to 23 points in different studies.

The cuff matters too. In a 2023 trial of 195 adults, using a regular cuff on someone who needed a large cuff overestimated systolic pressure by 4.8 points. For people who needed an extra large cuff, the difference was 19.5 points. A cuff that was too large could push the reading in the opposite direction.

You sit down, put on the cuff, and press the button. Simple enough. But what happened in the few minutes before that can change the result.

Taking a reading immediately after walking into the room, instead of resting first, raised systolic pressure by 4 to 12 points in studies. Talking during the measurement added 4 to 19 points to the top number and 5 to 14 to the bottom number.

Even needing to use the bathroom matters. A full bladder has raised systolic readings by anywhere from 4 to 33 points.

There is also a reason not to put too much weight on the first number you see. The first reading in a series is often 3 to 10 points higher than the readings that follow.

Blood pressure reacts quickly to what you are feeling.

The most familiar example is the white coat effect, when blood pressure rises in a medical setting. Across dozens of studies, the change in systolic pressure has ranged from a 13 point drop to a 27 point increase. The effect tends to be larger in older adults.

But it is not just doctors’ offices. Pain, an argument, a stressful deadline, or upsetting news can all temporarily push your blood pressure higher.

That reading is real for that moment. It just may not represent your usual blood pressure.

This is why readings outside the doctor’s office are so useful. Some people have high readings in the office but normal readings at home, a pattern known as white coat hypertension. Others have the opposite pattern. Their readings look normal in the office but run high at home. This is called masked hypertension, and it can be particularly important to detect.

Blood pressure rises while you exercise. That is a normal response.

It also means that climbing stairs to your appointment, rushing across a parking lot, or carrying groceries can raise a reading taken immediately afterward.

Then something interesting happens. After exercise, blood pressure often falls below its usual level for a while.

In a pooled analysis of 65 studies, blood pressure during the hours after exercise averaged about 5 points lower on the top number and 3 points lower on the bottom. This happened whether or not people had high blood pressure, whether or not they took blood pressure medicine, and regardless of the type of exercise.

Over time, regular exercise has an even more useful effect. It can lower resting blood pressure by roughly 4 to 8 points.

Unlike coffee or exercise, sodium usually changes blood pressure over days rather than minutes.

In a 2023 study of 213 adults between ages 50 and 75, one week on a low sodium diet produced systolic readings about 8 points lower than one week on a high sodium diet. Nearly three out of four participants responded, including people who were already taking blood pressure medicine.

That means a weekend filled with restaurant meals, soups, sauces, or cured meats may still show up in your readings on Monday. A stretch of lower sodium eating can move them in the other direction.

The Food and Drug Administration sets the daily value for sodium at 2,300 milligrams. The average American consumes about 3,400 milligrams a day.

Some foods add up quickly. A tablespoon of soy sauce contains roughly 900 milligrams. A whole can of some soups can provide most of a day’s sodium.

You do not need to memorize every number on the label. A useful rule is that 5 percent of the daily value or less per serving is considered low in sodium, while 20 percent or more is high.

Much of the sodium people eat comes from everyday foods such as breads, pizza, sandwiches, deli meats, cheese, soups, and restaurant meals.

Alcohol can make blood pressure readings especially confusing because its effect changes over time.

A 2020 Cochrane review of 32 trials found that one or two drinks lowered systolic pressure by about 6 points during the first six hours after drinking, even as heart rate increased.

With larger amounts, the pattern changed. Blood pressure dropped slightly at first, then rose about 4 over 2 points 13 hours or more later.

In practical terms, an evening reading after wine with dinner may look better than usual, while the next morning’s reading may look worse. Neither necessarily reflects your usual baseline.

Most of the people in these studies were healthy men, so the exact effect may differ for other groups.

Regular heavier drinking is a different issue. Over time, it can keep blood pressure higher. In pooled trials, people who drank more than two drinks a day lowered their blood pressure when they cut back. Those drinking six or more drinks a day who reduced their intake by half lowered systolic pressure by about 5.5 points.

And remember that a standard drink may be smaller than the glass in front of you. It is about 12 ounces of regular beer, 5 ounces of wine, or 1.5 ounces of spirits.

A cigarette, vape, or smokeless tobacco can affect your blood pressure almost immediately.

Nicotine tightens blood vessels and can raise systolic pressure by roughly 3 to 25 points for 20 to 30 minutes. That is why guidelines recommend avoiding tobacco for at least 30 minutes before a reading.

Some common medicines can matter too.

Ibuprofen, naproxen, and other NSAID pain relievers can cause the body to retain sodium. In a study of 444 people with arthritis, four months of ibuprofen raised 24 hour systolic pressure by 3.7 points. Among people who started with normal blood pressure, 23 percent developed hypertension, compared with 10 percent of those taking celecoxib.

Cold medicines deserve attention as well. Pseudoephedrine raised systolic pressure by about 1 point on average in pooled studies. That sounds small, but the effect was larger with higher doses and immediate release products, and some people may respond more strongly than the average suggests.

Steroids, some antidepressants, stimulants, and even black licorice can raise blood pressure too.

Yes, even the weather can show up on your blood pressure monitor.

Blood pressure tends to run higher in colder months. A review of 47 studies involving more than 850,000 people found that office readings averaged about 5.6 points higher on the top number and 3.3 points higher on the bottom in winter than in summer. Home readings showed a similar difference of about 6 over 3 points.

The seasonal change was larger in older adults and in people taking blood pressure medicine.

Cold causes blood vessels to narrow. Even the temperature of the room where you take your reading can matter. In studies, measuring blood pressure in a cold room raised systolic pressure by anywhere from 5 to 32 points.