Your thermostat may be working on your blood pressure.

New guidelines from the Japanese Society of Hypertension, published on October 10 in Hypertension Research, pull together years of research on cold homes. Cold indoor air pushes blood pressure up, especially on winter mornings. In one randomized trial, sleeping in a warm room lowered morning blood pressure by almost 6 points.

Most blood pressure advice ignores the room you wake up in. It shouldn’t.

Cold narrows your blood vessels and makes your heart push harder

When your body gets cold, it tries to keep heat in. Blood vessels near the skin narrow, so less warm blood reaches the surface. The body’s “fight or flight” nervous system also gets more active.

Narrower vessels and a revved-up nervous system mean your heart pushes against more resistance. Blood pressure rises.

That’s one reason blood pressure tends to be higher in winter. The guidelines note that winter is also the season with the highest overall risk of cardiovascular emergencies, including heart attacks, heart failure, some types of stroke and cardiac arrest.

The cold that matters most may not be outside. It may be in your bedroom.

Every degree colder indoors was linked to higher morning blood pressure

Several large Japanese studies measured people’s blood pressure at home along with the temperature of the room they were in.

  • 0.37 to 0.86 mmHg: how much higher morning systolic blood pressure (the top number) was for each 1°C drop indoors, across studies.
  • 8 degrees: the gap between a 12°C bedroom and a 20°C one. By those estimates, that’s roughly 3 to 7 extra points of morning blood pressure (our arithmetic).
  • 3 785 people: in the Smart Wellness Housing survey, those whose homes cooled more overnight, or swung more from day to day, had more variable blood pressure. Outdoor temperature swings showed no such link once other factors were accounted for.

These are observational studies. People in cold homes may differ in other ways.

That’s why the trials matter.

Warmer rooms lowered blood pressure in randomized trials

In one trial, 146 healthy adults were randomly assigned to spend winter nights in experimental rooms with either strong heating (about 24°C) or weak heating (about 14°C).

  • 5.8 mmHg lower morning systolic blood pressure in the warm rooms.
  • 14 vs 22 mmHg: the morning surge, the rise from the overnight low to the morning, in warm rooms against cold ones.

A second trial in older adults tested something you could do at home. Doctors gave participants instructions on how to use their heating. Their systolic blood pressure fell by 4.4 mmHg on average.

In both trials, the treatment was simply heat.

Your home is part of your heart health

Health doesn’t only live in your body. It also lives in the rooms around you.

Cold homes aren’t spread at random. The guidelines note that low household income, living alone and living in the same home for a long time were all linked to colder indoor temperatures. Cold homes also turn up in relatively warm regions, where houses may be poorly insulated.

That’s why the authors treat winter blood pressure as a job for homes, communities and government, not just doctors and patients.

The guidelines give you a simple winter routine

For winter, they recommend:

  • Keep indoor temperature at 18°C or above.
  • Keep temperature differences between rooms small, including the bedroom, dressing room and toilet.
  • Turn the heating on before you get out of bed, and avoid getting cold when you get up at night.
  • Warm the bathroom and dressing room before bathing, and avoid very hot or long baths.
  • Keep your feet warm with socks.
  • Measure your blood pressure at home, morning and evening.

Summer has the opposite trap. Blood pressure may fall during the day but stay high at night, so cutting medication based only on daytime readings can be a mistake. If you take blood pressure medication, talk to your doctor about seasonal changes rather than adjusting it yourself.

What the evidence can’t tell you

  • These are consensus guidelines that pull together existing research, not a new trial.
  • The trials show that warmer rooms lower blood pressure in the short term. There is no direct trial evidence yet that warming homes prevents heart attacks or strokes. The guidelines say such trials are hard to run, though long-term cohort data are building up.
  • Most of the research comes from Japan, where many homes are poorly insulated and heated room by room. The effect may differ in well-insulated, centrally heated homes.
  • The drop per person is modest. It matters because so many people are exposed, and because of the timing: winter mornings, when blood pressure naturally surges.

Warm the room before you get up

One of the easiest things you can do for your heart this winter is on the wall.

Set the heating to come on before your alarm, and keep your bedroom at 18°C or warmer.