Really short Bolus duration

I have seen similar type of behaviours and when I did a few basal tests I found that I was a tad short on the basal and that was impacting the time that the bolus appeared to last.

It might be worth doing some checks to see if your basal is correct.

Of course I should caveat my whole post with the fact that maybe the bolus does respond like that for you. What bolus insulin are you on?

/A

I am type 1 and my basal seems to change. I go 3 nights perfect and then a night with a 3am hypo. No changes to routine or sugar level before I sleep.

It's all rather complicated. If your basal level is not 100% spot on, your bolus appears to stop working as your background glucose production starts to counter the effects of it. The other thing to bear in mind is that the majority of the action of rapid insulin occurs in the first 2.5 hours after injection (look at the area under the curve in the below graph - 0-150 mins is about 85% of the area) so there isn't a huge amount remaining beyond that point. It best makes itself known if you exercise in that period. That's when you know you still have fast acting onboard.

I find the same thing. My bolus starts working very quickly and wherever I am at 2!hours is generally where I stay.
I inject 5 min before I start eating and if I don't eat I can feel it. I will also go low 1st if I don't eat within 5 min but I should also saybinam very low carb. Being so low carb I do gluconeogenisis quickly. Pretty much the same as carbs.

Wow. Auto correct did not understand me.

I take 1/2 unit novolog with my normal meals. I eat the same macros at each meal. I mix up proteins for variety. I take 1 unit lantus at 10:15 pm and like you I can't change that. I already tend to go slightly lower. I am sure I am out of it by morning as there have been times I didn't take it at all and went up about 20.
The best thing I did was find meals that satisfied me and then found my bolus. Recently I cut back on fat to drop a few pounds. I ended up having to cut my bolus in half as well as my basal. However what I think was really happening when I was taking 1 unit was that I was going low 1 st and then getting a liver dump. I always feel wonky when it's fluctuating and I feel so much better on 1/2 unit and my BS is much steadier. Another thing to consider is if you don't get breakfast right and you start the roller coaster it won't end until you sleep it off , at least in my case. My BF has to be VLC and protein and higher fat. So I have one piece of romaine lettuce with a small piece of deli turkey and mustard and mayo or guacamole and celery. I can ramp up protein and carbs ( but still just above ground veggies) and meal size as the day goes on with the same bolus. I do weigh my protein still as that raises me like carbs. Maybe your macros in your meals are causing your reaction. If I don't have enough fat it throws things off. I have to balance all fat proteins and carbs to the meal size. If I eat more of one I need to eat more of the others. Yes, I have done tons of testing to figure this out but I can't stand fluctuation. It exhausts me.
Sounds like your basal is ok but your meals are off. I say this because when I was eating carbs, even small amounts I couldn't keep anything stready. Down 1st then up and stayed up. Injected insulin is opposite of the natural body. Getting insulin there at the same time as the food is a science in itself. I could never do it with carbs other than veggies. I have to keep the same amount of veggies, fat and protein at each meal and that is what works for me. Most would not go through the hassle but I feel so much better and now I have my meal plan and bolus so I'm sticking with it.
That was windy haha

If it is your basal that's off, you may find a pump would make a huge difference. I could never sort my basal and my blood sugar was up and down (I only have a small amount of basal) but the pump allowed me to fine tune my basal to my needs. It was a revelation - proper control for the first time

strange I cannot have a pump. yet my basal is always changing. I need a different dose when not working as to when I am working. How that can be done, as I have been put on Toujeo which takes at least 3 days to change the dose. No one cares or offer a solution. Wish i never worked or contributed to the nhs. After 50 years my consultant is not interested. Just likes to give them to young people.