Overnight Lows After Evening Exercise

The most common serious problem from exercise in Type 1 is not a low during the activity. It is a low six or eight hours later, in the middle of the night.

The reason is that your muscles do not just use glucose while you are moving. Afterwards they restock the glucose they store, drawing from your blood to do it, and that process continues for hours. Evening exercise puts that window squarely across your sleep.

It is also the worst time for it, because a night-time low may not wake you. It shows up instead as nightmares, waking sweating, or feeling confused and unwell the next morning.

So the practical rules for evening activity are these. Check before bed rather than going to sleep on the last reading you happen to have. Treat a bedtime reading at the low end of your range as a reason to eat something, even if you feel fine. Ask your doctor whether your background insulin or bedtime snack should change on days you exercise.

If you have a sensor, this is precisely what its overnight alarms are for, and a night after unusual activity is a good reason to set them tighter.

If you wake with a high reading after evening exercise, do not automatically increase your night insulin — it may be a rebound from a low you slept through.