# Sleep as Immune Training: what really happens at night > Your immune system sorts itself out while you sleep. What happens at night, why the sleep cut-off matters, and a seven-day protocol. One night with four hours of sleep, you notice the next day. What you do not notice: your immune system notices it too. If you want fewer colds this autumn, think less about vitamin tablets and more about the time the lights go out. ## What happens in the immune system at night Sleep is not time off for the body, it is a shift with different staff. While you sleep, activity shifts from muscles and digestion towards repair and defence. Three things happen. First: in the first half of the night, during deep sleep, the release of growth hormone rises. It supports tissue repair and cell building. Second: T cells, the specialists of the defence, migrate to the lymph nodes in greater numbers at night, where they exchange information about pathogens. Researchers link sleep to the formation of immunological memory, meaning the ability to recognise a pathogen faster on second contact. Third: the level of the stress hormone cortisol is at its lowest at night. Cortisol dampens the immune response. Its nightly low gives the defence room to work. Shorten the night and you shorten all three processes. Studies link short sleep of under six hours to a markedly higher susceptibility to colds than sleep of seven hours and more. The response to vaccinations also turns out weaker after sleep deprivation in research. ## Why the cut-off matters more than the duration Most people think of sleep in hours. The body thinks in rhythm. Your internal clock controls when melatonin rises, when body temperature drops, when growth hormone is released. It works best when it gets the same signal every day. A fixed time for "lights out" is that signal. If you go to bed at 23:00 on weekdays and at 01:30 at the weekend, you shift your internal clock by two and a half hours, the equivalent of a flight from Zurich to Dubai, every week. The term for this is social jetlag. It costs deep sleep and with it exactly the phases in which the immune system is most active. That makes the sleep cut-off the most important lever: a time that is not negotiable, seven days a week. It is not the hours that count first, but the regularity. ## The seven-day protocol The protocol is deliberately simple. It has three phases per evening and needs no equipment. **60 minutes before the cut-off: winding down** - Dim the lights. Ceiling lights off, only table lamps or candles. Bright light in the evening suppresses melatonin. - Last meal by now at the latest. Full digestion keeps body temperature high and delays deep sleep. - Air the room and bring it to 17 to 19 degrees. **30 minutes before the cut-off: screens off** - Phone in another room or in flight mode. Not "just a quick look". - Instead: a book, a notebook, a short conversation. If you need to clear your head, write three things for tomorrow on a sheet and put it away. - Warm shower. It widens the blood vessels in skin and hands and then speeds up the drop in core temperature. **The cut-off: lights out** - For most people between 22:00 and 23:00. Choose a time you can hold on seven days, not the most ideal one. - Alarm in the morning fixed as well, weekends included. Maximum 30 minutes deviation. - Not asleep after 20 minutes: get up, read in dim light, go back when tiredness comes. The bed stays reserved for sleep. **In the morning** - Within 30 minutes of waking, go outside for 10 to 20 minutes of daylight. That sets the internal clock and makes the next cut-off easier. - Caffeine only after 90 minutes, last coffee eight hours before the cut-off. ## What you notice after a week The first night usually does not feel better. From day three or four, most people report falling asleep faster and waking less during the night. After seven days, the time to fall asleep is often markedly shorter and the morning less sluggish. For the immune system, the week is the beginning. The effect builds up over weeks. If you hold the cut-off through the whole autumn, you give your defence the same time window for its work every night. A tip for reviewing it: every morning, note in one sentence when the lights went out and how you slept. After seven days you see on a sheet of paper the pattern that no tracker shows you. ## The most common disruptors Alcohol: a glass of wine makes you tired, but sleep becomes shallower and the second half of the night restless. On the seven protocol days, best to leave it out entirely. The warm bedroom: above 20 degrees, the drop in core temperature the body needs to fall asleep is delayed. A tilted window or a thinner blanket is often enough. If you are cold, take socks, not the heating. Late training: intense sessions after 20:00 keep pulse and temperature high. Easy movement is fine, hard intervals belong in the morning or at midday. The weekend: the cut-off's greatest enemy. Sleep two hours later on Friday and Saturday and you start Monday with jetlag. A compromise: maximum one hour deviation. Rumination: if you lie awake at night planning, get a notebook in the evening. What is on paper, the head no longer has to hold on to. ## Frequently Asked Questions **Are six hours of sleep enough if they are regular?** For most adults, no. Seven to nine hours is considered the target range. Regularity is the first step, duration the second. If you hold the cut-off and are still tired, move it 30 minutes earlier. **What if I work shifts?** Then the protocol applies to your personal evening, whatever the clock says. What matters is regularity within a shift block and darkness in the bedroom, with blackout blinds and a sleep mask if needed. **Do sleeping pills or melatonin help?** Melatonin can be useful for time zone changes but does not replace a rhythm. Sleeping pills belong in medical hands. For immune training, natural deep sleep counts, and you get that through light, timing and temperature.