Without knowing what your insulation duration tests (if you have done them) have shown, I would say a combo of both. It seems to be your overnight heading into that track is not an overnight track I would desire. I would deftinately reduce basal in that range, unless you were heading down from a correction from midnight to 2 am.
During the day bolus changes for us occur after 3/4 days of consistent action. i.e. am I going low at the same time for 3/4 days and it isn’t explained by a meal overdose.
Basal is exactly what your body needs, nothing more or less. It really is different for everyone, and comparing to others is only of limited value. Over the last three years we have seen my son’s basal needs change over time, so it is correct for that period of time that it works, and then it needs to be reevaluated to ensure it is still the right amount.
What we do to determine insulin duration is take a high (not a 300,more like a 200) then we treat with insulin and skip the next meal. Note when the insulin doesn’t appear to be lowering your blood sugar anymore. Do this 2 or 3 times and you should be able to get a good estimate for your personal insulin duration.
If you are using a pump, there are some other things you can do, i.e. play with the duration settings and see what works best for you, but this is a little more complicated.
Honestly either is possible. The problem is you only have 4.5 hours between events. If you stretch that to 6 hours you can be reasonably sure it was the insulin you injected just then. For my son, if he hasn’t been exercising it usually takes 45 minutes before a flat line turns downward. Some on here have reported as quickly as 30 minutes, and as long as 1.5 hours. So clearly there are pretty large differences.
Testing is the only way you will tease out the variables.
Also if my son has exercised in the 2 hours prior to the meal bolus, it will take effect much faster. like 20-30 minutes.
That’s almost a meaningless question, in that your basal is whatever your body needs, so any preconceived notion of what it “should be” or what a “reasonable value would be” is pretty much irrelevant. In the graph you show, your BG is dropping starting at 2pm, and it continues doing that until 8pm (with two spikes presumably from carbs.)
For myself, I need about 0.7u/hr overnight, and 0.2u/hr in the afternoon. If I took 0.5u basal in the afternoon I’d be crashing hard.
It might be reasonable to try reducing your afternoon basal (starting around noon or 12:30 — a couple hours before the drop starts) by a small amount to see if that tends to make your BG drop less fast during the afternoon, and every second or third day reduce it again until you find a basal that doesn’t seem to make your BG drop or rise.