Not all days are the same. More physical activity generally means less insulin needed, while more stress often leads to more insulin required.
Auto mode adjusts insulin needs to match the requirements of the moment, but I’ve found that auto mode doesn’t do a great job of increasing insulin for the days where I need a lot more. Auto mode is cautious about increases, which is understandable. Auto mode doesn’t know that it’s a “work day” or a “sick day.”
Therefore, I’ve started using manual mode some of the time, particularly on days where I’m needing more insulin than normal.
My insulin pump (a Medtronic Minimed 670G) has ways to set up different basal rates for different kinds of days. They have an option to set a “work day” rate. For me, a work day is full of meetings and sitting in front of the computer. The higher the stress, the more insulin I need. So I have a work day basal rate that’s higher than normal.
If your work day consists of a lot of physical activity, then your work day basal rate would be lower than a normal day.
I tend to need more insulin on work days, especially in the morning, so I’ve developed a work day habit of switching into manual mode. I set the “suspend before low” setting so that if I’ve overestimated the insulin amount, the pump will halt the basal flow when my blood glucose is dropping. (My pump requires me to set this suspend before low setting every time I switch into manual mode, as it doesn’t remember the setting, unfortunately. This is a small annoyance.)
This sometimes leads to the pump suspending the insulin before lunch time, but then I’m usually at around 100 mg/dL and it’s lunch time, so I resume basal, bolus, and then eat lunch. That’s a much better result than being at 200 mg/dL at lunch time from all of the work day related stress and an auto mode that hasn’t been able to keep up.