What a hypo does to your body
What is happening inside a low: why it feels so rough, what the shakes and sweats are for, and how treating it works.
A hypo is when your blood glucose drops too low, below 4 mmol/L, and your body starts sounding alarms to get it back up. It feels rough because your brain runs on glucose and it’s suddenly running short. It passes once you treat it, and treating it is routine, not a crisis, most of the time.
- A hypo (hypoglycaemia) is a glucose reading below 4 mmol/L, and it moves in one direction: too low.
- Early signs come from adrenaline: shaking, sweating, a racing heart, sudden hunger or irritability.
- If it keeps dropping, your brain struggles: confusion, slurred words, trouble concentrating, and eventually you can’t treat yourself.
- You treat it with 15 to 20g of fast-acting carbohydrate (sugary drink, glucose tablets), wait 10 to 15 minutes, then recheck.
- A low can hit at the time, or arrive hours later, including overnight, after exercise, alcohol or insulin still working from earlier.
What’s happening while it’s happening
Your brain doesn’t store much glucose, so it needs a steady supply from your blood. When that supply drops, your body treats it as an emergency and reacts fast, before you’ve even worked out what’s going on.
First it releases adrenaline. That’s the shaking, the sweat, the thumping heart, the sudden anxiety or hunger. Those symptoms aren’t the low itself, they’re the alarm. Your liver also tries to help by releasing its own stored glucose, which is why a hypo sometimes seems to lift a little on its own before you’ve treated it. That’s not a reason to skip treating it.
If glucose keeps falling, the brain itself starts to struggle. This is where thinking gets slow, words come out wrong, and judgement goes, including the judgement you’d need to notice you’re low and do something about it. That’s the point of treating early, while you still can.
A low doesn’t only happen when you’d expect it
Plenty of hypos hit at the obvious moment: during or straight after activity, or an hour or two after a meal when the insulin you dosed is working harder than the food behind it. That’s when a low is most likely, so it’s also when you’re most worth watching. A brisk walk, a workout, even a busy day of being on your feet can tip you down while it’s happening.
The part that catches people out is the delayed one. After exercise, or after alcohol, or with rapid insulin still active from earlier, glucose can keep drifting down for hours, right through the night. Your muscles pull in glucose to refill their stores long after you’ve stopped moving, and alcohol blunts the liver’s usual rescue. That’s why a low at 2am can trace back to a run at 6pm. If you’ve been active or had a drink, a check before bed and a sense of whether you’re trending down is worth more than a single number.
Why you feel wrecked afterwards
Once you’ve treated a hypo and your glucose is back up, you don’t necessarily feel fine straight away. A lot of people feel drained, headachy, shaky or a bit foggy for a few hours. That’s the aftermath of the adrenaline surge and the effort your body just put in, not a sign anything’s still wrong.
There’s also a rebound to watch for. After a low, glucose can swing high, partly from the fast carbs, partly from the liver’s response and any panic-eating on top. A single hypo can leave your numbers unsettled for the rest of the day. That’s the body correcting hard in both directions, and it usually settles.
What repeated lows do over time
One hypo, treated, does no lasting harm. The thing worth knowing is what happens when lows keep coming. If you’re going low often, your body can stop raising the alarm. The adrenaline warning gets quieter, so a low sneaks up with no shaking or sweating, and you’re already confused before you’ve noticed. This is called hypo unawareness.
It’s not permanent and it’s not a character flaw. It’s your body adapting to something it now sees as normal. Avoiding lows for a stretch of time can bring the warning signs back. If you’ve noticed you no longer feel your hypos coming, that’s a specific thing to raise with your diabetes team, because it also affects driving.
What changes how a hypo feels and how fast it hits
Two hypos at the same number can feel completely different. A slow drift down to 3.8 can feel milder than a fast plunge from 12 to 5, even though 5 isn’t technically a hypo, because your body reacts to the speed as much as the level. Alcohol, recent activity and insulin still active in your body all shift how a low behaves, how long it lingers, and how late it can arrive. None of this means you’re doing it wrong, it means the same event doesn’t repeat identically.
What a hypo can look like
Examples, not instructions or doses.
You dose for a meal, then end up more active than planned. Within the hour you’re shaking, sweating and suddenly starving. A reading shows 3.4 mmol/L. Fast carbs and a 15-minute wait, and it lifts, but you feel flat for the rest of the afternoon.
You had a long walk in the evening and went to bed at 7 mmol/L feeling fine. At 3am you wake sweating at 3.2. The muscles you worked hours ago were still pulling in glucose to refill. This is the delayed low, and it’s the one people don’t see coming.
You catch a 3.6 on your monitor with almost no warning signs, no shaking, no sweat. If that’s happening regularly, it’s worth flagging: the missing warning is the part that matters, more than the number.
Worth paying attention to
Questions that make an appointment useful
When it's urgent
If someone with a hypo becomes confused, unable to swallow, unconscious or is having a seizure, do not put food or drink in their mouth. This is a severe hypo and they cannot treat it themselves. Use glucagon if it is available and you know how, and call 999. Once they have recovered enough to swallow safely, give fast carbs then something slower. If you are ever unsure whether a low is turning serious, ring NHS 111, and your diabetes team can review what led up to it afterwards.