Why your blood pressure at night matters more than during the day

If you measure at home, you capture the morning and the evening. The time window that tells you the most lies in between: the night.

Blood pressure follows a 24-hour rhythm. It rises steeply in the early morning hours, stays on a plateau during the day and falls to its lowest level of the day during sleep. This nocturnal dip is not a minor detail but a recovery phase for the heart, blood vessels and kidneys.

Dipper, non-dipper, reverse dipper

Medicine distinguishes patterns by the extent of the night-time fall compared with the daytime average:

• Dipper – a fall of 10 to 20 per cent. This is the normal pattern.
• Non-dipper – a fall of less than 10 per cent. Pressure stays close to daytime levels at night.
• Extreme dipper – a fall of more than 20 per cent. This can also be unfavourable, because blood flow drops too much in the low phase.
• Reverse dipper – blood pressure is higher at night than during the day. This pattern is associated with the highest risk.

The key point: night-time blood pressure predicts cardiovascular risk better than the daytime value – and better than the reading taken at the doctor's surgery. This is one of the reasons why 24-hour monitoring carries so much weight in the guidelines.

The pulse usually falls at night as well. Why the two do not necessarily move in step is explained on Palpitations, pulse and blood pressure.

Where the thresholds lie

24-hour monitoring has its own thresholds – lower than the 140/90 mmHg used in the surgery:

Period	Threshold (average)
24-hour average	from 130/80 mmHg
Daytime average (awake)	from 135/85 mmHg
Night-time average (asleep)	from 120/70 mmHg

You can find how the other thresholds are classified in the blood pressure values table. A night-time average of 125/75 mmHg sounds harmless but is above the threshold.

Besides the level of these averages, it also matters how much time the pressure spends within the target range overall – with different corridors applying by day and at night. Why this time span carries its own information is explained in Blood pressure target range: why the time within it counts.

What prevents the night-time dip

Obstructive sleep apnoea. The most common and most frequently overlooked cause. Every pause in breathing triggers an arousal response that drives pressure up. Covered in detail in Sleep apnoea and high blood pressure.

Kidney disease. Impaired kidney function is often associated with a non-dipper profile – in both directions: the kidney suffers from the missing dip and at the same time contributes to it.

Diabetes mellitus and associated damage to the autonomic nervous system.

Shift work and a disturbed sleep-wake rhythm. The internal body clock helps control the dip; if you live against it, you partly lose it.

High salt intake – especially in salt-sensitive people; the size of the effect is described in Salt and blood pressure.

Timing of medication. Whether taking medication in the evening improves the night-time profile is debated among experts; large studies have produced differing results. So do not change the time you take your medication on your own, only after consulting your doctor.

Why home measurement does not show this

A non-dipper profile is invisible with morning and evening measurements. Both values can be within the target range while pressure stays high throughout the night. This is exactly the blind spot of home measurement.

You can draw two indirect clues from your log:

• Morning values clearly above evening values – consistent with a pronounced morning rise from a high night-time level.
• A value that does not reach the target range with medication, even though the daytime values look good.

Either is a reason to ask your doctor about 24-hour monitoring – no more, but no less. How to set up home measurement properly is explained in When should you measure blood pressure?.

How 24-hour monitoring works

For 24 hours you wear a cuff on your upper arm, connected to a small recorder on your belt. During the day the device typically measures every 15 to 20 minutes, at night every 30 minutes.

To make sure the analysis is accurate, you keep a short diary: time of getting up and going to bed, medication, physical exertion, unusual events. Without your bedtime, the night-time average cannot be defined properly.

Do not move your arm during a measurement and keep it hanging still. At night many people are bothered by the cuff – this is normal and does not make the measurement worthless, as long as enough valid readings are collected.

When you should seek medical advice

• If your morning values are regularly clearly above your evening values
• If your blood pressure does not reach the target range despite medication
• If you snore loudly with observed pauses in breathing
• If you have known kidney disease or diabetes – here 24-hour monitoring is particularly informative
• If you need to urinate at night together with raised values

In case of sudden severe headache, chest pain, speech or vision problems or one-sided paralysis: call the emergency services on 112 immediately (the European emergency number).