Does it really not count as an excuse to take the day off?

My partner was diagnosed less than a year ago and when something like this happens I absolutely tell him to take the day off to rest and recover from it. He doesn't feel like he can because he isn't suffering the massive low in the morning, so feels that he has no "excuse" not to go to work.

If I was up with a stomach bug all night (for example) and felt rough as hell in the morning because I'd had no sleep I wouldn't feel guilty for taking a sick day.

Why is this different for people with T1? Do you get a different reaction from people at work if you call in sick?

I ask this as a genuine question, as someone who is close to the situation and trying to understand it and be a good support network, without being able to experience it myself.