Once the dinner things are put away, the kitchen falls quiet again. Water poured into the teapot sends toasty steam loosening into the night air. Hojicha looks like a gentle tea, even before bed. Yet that cup, too, holds caffeine.

Below: the amount in a cup, the amounts that bear on sleep, the time of drinking, and evening swaps.

Toastiness, and what the leaf holds

The brown in the cup is the colour of roasted leaves. Even roasted, hojicha keeps the caffeine of the tea leaf.

Japan's national sleep guide says the caffeine in teas is held in the tea leaf. Even a dark roast lost only about a tenth in a prefectural trial. Fire made the toastiness, but it barely touched the caffeine.

The sources say this much: hojicha has caffeine too. How much roasting removes is traced in Does roasting reduce caffeine in hojicha?. Then there are the amounts that reach sleep.

Amounts that reach sleep

The bedside clock ticks off the seconds. What bears on sleep is less the single cup than the amount piled up over a day.

Japan's national sleep guide says a large daily total can affect night-time sleep even when taken in the morning. In studies measured by brain waves, more caffeine meant less deep sleep and more waking in the night. One overseas assessment, too, finds that a single amount just before bed can disturb sleep in some adults.

With hojicha in a teacup, it takes a little over three cups to reach that single amount [calculated].

Sensitivity, though, varies widely between people. After amount comes time.

A gentle slope from evening

The west window turns slowly to a deep orange-red. Caffeine leaves the body a little at a time.

Like the evening shadows lengthening, the amount left in the body thins with time. In the guide's example, which assumes a mid-range clearance rate, half of what was taken at dinner time is still there at midnight. So the guide advises cutting back from the evening on.

How fast it clears varies a good deal between people. The evening cup can also be swapped for another drink.

Barley tea in the kettle, the evening cup

In the fridge, a jug of barley tea brewed at midday is chilling. Barley tea, made without tea leaves, contains no caffeine.

Japan's national sleep guide also lists swaps such as barley tea, buckwheat tea, black soybean tea and corn tea. How barley is roasted is in Mugicha, sobacha and genmaicha. Hojicha by day and barley tea at night is one way to switch by the hour.

The guide stops at examples of swaps. Caffeine across drinks is set out in one table in Hojicha caffeine: when, and for whom?. The sound of water rinsing the cup carries softly through the night kitchen.

The numbers in detail

Daily total
The MHLW Sleep Guide for Health Promotion 2023 says that above 400 mg of caffeine a day, sleep may be adversely affected whenever in the day it is taken.
From the evening
The same guide says 100 mg or more from the evening on brings difficulty falling asleep, less slow-wave (deep) sleep and more waking during the night.
Half-life
Blood half-life in Japanese people varies from 3 to 7 hours. The guide's worked example: with a 5-hour half-life, 100 mg taken at 7 p.m. leaves 50 mg at midnight.
Timing study
A systematic review of adults aged 18–65, cited in the guide. Above 107 mg, night-time sleep was affected even about 9 hours before bed; above 217.5 mg, even about 13 hours before.
Just before bed
The Austrian Agency for Health and Food Safety (2017) says a single 100 mg just before bed (about 1.4 mg/kg at 70 kg) may disturb sleep in some adults, and that similar effects may occur in children. This is as presented in the Food Safety Commission of Japan's fact sheet. EFSA, too, says a single 100 mg may affect sleep, especially before bed.
One cup
Hojicha infusion holds 20 mg per 100 mL (15 g of leaf, 650 mL of water at 90°C, 0.5 minutes), so a 150 mL cup holds about 30 mg. Reaching 100 mg takes about 3.3 cups [calculated].
Roasting
In a Kochi Agricultural Research Center Tea Experiment Station trial (2014 data), caffeine in the dark roast was 6–13% lower than in the unroasted tea.
Swaps
Because the caffeine in teas comes from the tea leaf (Camellia sinensis), the sleep guide lists drinks made without tea leaves as swaps: barley tea, buckwheat tea, black soybean tea, corn tea and caffeine-free herbal teas.