Your blood pressure has a calendar. Track it for a year and the seasons show up in the numbers, reliably enough that researchers have measured the size of the swing across nearly a million people.

The effect is larger than most everyday advice implies, and one part of it runs in the opposite direction to the rest, which is the detail worth knowing.

Key takeaways

• Blood pressure is lower in summer than winter by about 5.6/3.3 mmHg on office readings and 6.1/3.1 mmHg on home readings, pooled across 47 studies and 856,539 people (Kollias et al. 2020, Journal of Hypertension).

• Night-time pressure moves the other way, running about 1.3 mmHg higher in summer.

• The swing is larger in older people and in those treated for hypertension.

• A seasonal shift of this size is comparable to what several lifestyle changes achieve, which is why comparing January with July tells you very little.

How big is the seasonal swing?

Bigger than the difference most habits make, which is the surprising part.

A meta-analysis pooled 47 studies covering 856,539 participants and compared summer with winter across four different measurement methods (Kollias et al. 2020):

A separate pooling of 28 prospective studies that followed the same individuals across seasons found the same thing, slightly larger: office readings −5.8/−4.0 and home readings −6.4/−3.3.

That last point matters. These are not different people measured in different months; they are the same bodies in January and July. The swing is real and it happens to you personally.

For scale, a 5 to 6 mmHg seasonal drift is comparable to what most blood pressure habits are worth individually. A sodium reduction of 3,000 mg a day is worth about 4.3 mmHg. So the difference between your winter and summer readings can exceed the effect of a change you worked hard at.

Why does night-time run the other way?

This is the finding that breaks the simple story, and it was consistent enough that the authors flagged it explicitly.

Daytime pressure falls in summer. Night-time pressure rises slightly, by about 1.3 mmHg systolic. Every daytime measurement method agreed with each other and disagreed with the night-time one.

The usual explanation for the daytime pattern is temperature: cold narrows the small vessels near the skin to conserve heat, which raises pressure, and warmth relaxes them. Overnight, when you are under bedding in a controlled indoor environment, that outdoor exposure largely disappears, and warm summer nights may disturb sleep in ways that push pressure the other way.

The practical version: your daytime readings carry nearly all of the seasonal signal, and the night-time reversal is small enough that you would never spot it in your own data.

Who sees the biggest swing?

Two groups, and both are worth knowing about.

The seasonal difference was larger in people being treated for hypertension and larger in older individuals. In the meta-regression, the size of the daytime ambulatory swing was significantly related to the proportion of treated hypertensive patients in a study, and borderline related to age.

This has a real consequence. If you are on treatment, the same dose meets a different seasonal baseline in February than in August. That is a conversation for a clinician rather than something to act on yourself, but it is a reason to bring a full year of readings to an appointment rather than a fortnight.

What does this mean for reading your own numbers?

Mostly it means being careful about what you compare with what.

Compare like months, not adjacent ones. Your February average against last February is informative. Your February average against last August is mostly a temperature reading.

A winter rise is not a deterioration. If your readings drift up in the dark months, the most likely explanation is the season, alongside the habits that travel with it: less movement, saltier food, more alcohol, disrupted sleep. That cluster is also why resting heart rate peaks in the first week of January and bottoms out at the end of July.

Judge changes over weeks, not days. Single readings are noisy enough that a seasonal drift of 5 mmHg is invisible in any one morning. A multi-week average that has clearly settled somewhere new is the signal.

Keep the log. The genuinely useful thing an app does here is hold a year of properly taken readings in a form you can show someone. It is not useful for deciding what a year of readings means.

Is a seasonal pattern something to worry about?

On its own, no. It is the expected behaviour of a system that responds to its environment, documented across nearly a million people.

What is worth attention is a sustained shift that does not match the season: a multi-week average that has moved clearly away from your usual range and stayed there, especially if it has moved up in summer or failed to come down. That is the pattern a long home log is genuinely good at surfacing, and it is worth raising with a clinician rather than interpreting alone.

The same applies to readings that simply concern you. A year of rested home readings gives a clinician far more to work with than any single number ever could, and it removes the white coat problem from the conversation entirely.

Frequently asked questions

Is blood pressure higher in winter?

Yes, on daytime readings. Pooled across 47 studies and 856,539 people, summer readings were about 6.1/3.1 mmHg lower than winter on home measurement and 5.6/3.3 lower in the office (Kollias et al. 2020). Night-time readings were the exception, running about 1.3 mmHg higher in summer.

How much can blood pressure vary through the year?

For most people, roughly 5 to 6 mmHg systolic between the coldest and warmest months, and more in older people and those on treatment. That is comparable to the effect of several deliberate lifestyle changes, which is why seasonal comparisons are so easy to misread.

Why is my blood pressure higher in cold weather?

Cold causes the small blood vessels near the skin to narrow in order to conserve heat, which raises pressure. Winter habits compound it: less activity, saltier food, more alcohol and disrupted sleep all cluster in the same months.

Should I change anything because of the season?

Not on your own. If you are on blood pressure treatment, a seasonal shift in your baseline is worth showing a clinician, since the effect was larger in treated patients. Bring the year of readings rather than a fortnight, and let the trend do the talking.

How long should I track before a change means something?

Weeks, not days. Day-to-day scatter in blood pressure is large enough to hide a seasonal-sized effect entirely. A multi-week average that has clearly settled outside your usual range is information; a high Tuesday is weather, sometimes literally.

Knit shows patterns, not diagnoses. Talk to a clinician about readings that concern you.