Interpreting overnight basal test

I’m in the process of establishing all of my pump settings (starting with basal rates, then ISF, then I:CR).
I basal tested my overnight basals last night; to give you all the info, I finished exercising (ran 1.25 mi) at 6:35 pm, ate a burger (5 oz, no bun) and 1 cup corn at 6:55, dual waved bolus at 6:45 and the second half at 7:15, went low at 7:45 so I treated with glucose tabs (which account for that rise at 8). I know this isn’t a perfect basal test and am planning on retesting tonight. Here are the results from last night (3 images showing three BG values at 10PM, 3AM, 2AM):

I confirmed the g6 sensors reading with finger checks and my guardian sensor read similar trends.

I know I need to repeat the test but I’m comfortable making changes to the rates and then seeing how that looks tonight. So - I was wondering if I could have help interpreting the test and understanding which rates to adjust. I’m on FIASP which takes 5-15 minutes to kick in for me and I have an AIT of 2hr15-2hr30min.

  • So using those times, do I adjust the basals that are 15 min ahead of the trends? An hour ahead of the trends?
  • How much do I adjust by? My CDE and endo seem to like increments of .1; is this based off of my ISF (which I don’t believe to be accurate (am planning on testing it once I don’t have basals as a confounding variable))?

Overnight basals are currently:
|8p-10p: 1|
|10p-12a: 1|
|12a-2a: 0.85|
|2a-3a: 0.85|
|3a-6a: 0.9|

I’m thinking the 10p-12a needs to be decreased, and the 12a-2a maybe change to 12a-1:30 and decrease then 1:30-3am increase?

I am planning on reaching out to my CDE and endo additionally but appreciate your help and explanations ahead of time!!!


@LarissaW, I was quite dense for a while—couldn’t understand the three curves. Now I understand that you showed all three pics to display the three BG values…

AIT- I assume you mean DIA, Duration of Insulin Activation? If so, your DIA appears surprisingly low by multiple hours. Many people, we included, use small DIAs in their pump programming, but it really is to fool the program. I would be really surprised, even with FIASP, if you are below 4 hours (the later hours’ effect is of course weaker)—although everything is possible of course! As an example, we calculate 5:15-5:30 for us, but, if we use very high boluses such as 10U, we see effects even beyond 6 hours.

Does your basal really take 5-15mns to kick in? This is remarkably fast, although possible with FIASP. @Thomas, does this match your experience? I know you had seen amazingly fast activation time.

If you are convinced that your insulin activation time is 15 mins, then yes—you would make changes at least that long ahead. You would want to lower your 10-1:30 or 10-2 by a bit, either 0.05 or 0.1, then increase your 1:30-3 or 2-3:30 some. But you have to be careful: these times may easily change. I don’t like to have a short window of adjustment in the middle of the night, because this is quite sensitive to time, and to your schedule, cycle and activities. I’d much rather have a bit more tolerance on the BG levels, and have a less sensitive time trigger. For instance, maybe you can have a longer segment in the middle that is not quite as high as a short one would be?

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O yes, sorry about that; I should’ve clarified.

I guess so? AIT being active insulin time, which is the only way I’ve heard it referred to, but they sound like the same thing. :sweat_smile:
And regarding the timing on how long my insulin is active: I had done a “test” of this as instructed by my CDE before that went as follows - give a correction for elevated blood sugar and measure the time it takes for the blood sugar to fall and stabilize for 30 mins. I did this without any other variables like exercise, food or stress and found that for me on FIASP 2 and a half hours was the timing on how long the insulin was active for me. Have I interpreted what my CDE instructed me on correctly? Maybe this is just the timing I use to “fool the programming”?

I’m not sure how to gadge how long it takes for basal to kick in, but it takes 5-15 minutes for a bolus to kick in (even small ones).

That totally makes sense on the variability of timing and I hadn’t thought of it. And you mean here like a shallower increase in basal but over a longer time period?

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If we do the same test as you do, we get about the same results, because we don’t really notice what happens later. But, if we let it run for a lot longer without taking insulin or carbs, we see a much longer tail effect. Then when the action is “totally” over, we always see a little squiggle on the BG curve, typically a small step up, the sign that there really is no insulin left. This is particularly sensitive when you stack multiple times, because the tail becomes more critical: you can run low for a lot more of the tail.

For us, for instance, any correction is largely over in 2.5 hours, but we can run low from a stack 5.5 hours (or more) after the last injection.

So we use these two numbers for different purposes.


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@Larissa, what happened at 2:00 am, was an intervention required or was it just a change in your body’s need for insulin?

Honestly, because your basals look to be really close to what you need, i.e. we would love to have that line at night. I would see if you do the same for the next couple of nights before fine tuning.

Because your trend is pretty close to perfect, I would go with your CDE’s advice about rate of switching.

Finally, what is your goal? A completely flat line, or a line that leans towards keeping your bg low?

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Adding this to my list of things to test!

No treatment and no suspends. I think just a change in my body’s need for insulin.

Like you’re suggesting, I think I’ll get one more night of this “baseline” basal test then start tweaking the following night. sigh it’s so stressful exercising at eating all before 7

Hmmm I think flatline is the goal here. I don’t like that I trend lower throughout the early part of the night because I’d like to go to bed at 100 without fear of waking up with the sweats and alarms going off (which has been happening)

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My first change would be to the 10 - 12 basal. If tonight goes with the same level of bs decline, then I might change by 0.02 or 0.03, the reason is that it is a pretty big change to go from 1 to 0.85 at midnight.

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If this were a trend I’d suggest slightly decreasing the 9-10/11pm basal, but it’s impossible to know from one night’s line how much of the 10:30-2:30 gradual decline is a result of too much basal or too much dinner bolus or the type of meal you ate or a delayed response to exercise.

As you know, you could do exactly the same thing tonight and get radically different results, so making changes to basal based on one night’s results is problematic because you don’t know exactly what you’re addressing or what you’re trying to fix (see the four variables above and add another dozen variables, such as stress, quality of sleep, time of the month, etc.). Personally I’d give it a week and then see if there’s a trend – and make sure that trend is related to basal and not mealtime bolus or timing of exercise or whatever.

as in, with different meals/mealtimes and different exercise times if there’s still the trend THEN we can assume it’s because of basal?

The Insulin Duration (Tandem phrase for it) is set to 2-1/2 hrs on our X2 pump with Fiasp. This is not fooling anything. This is what we believe is the proper value. I am sure chemistry analysis of bloodwork would still see insulin active but that is far past my concern. Once the curve on the cgm graph levels out then I consider the insulin to be done for all practical purposes.



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@LarissaW, overall, I subscribe to the idea that you want your basal rates to keep you within a 30 point bg range if possible. Due to the dinner, exercise, dual wave, and glucose tabs, I’d be hesitant to conclude anything about your basals prior to that 2:10AM dip/rise. And your total climb in the span of about an hour at 2AM is just 39 points. I’d call that close enough if we’re just looking at this one night. Even with Fiasp being quick in and quick out, I’d personally still want to skip dinner or do a really early dinner to get the cleanest data possible for overnight basal testing.

One other thought…and this is pretty down in the weeds but it is my biggest contributing factor to my blood sugar stability since going off of birth control years ago (and it was still in play to a smaller extent on birth control)…when you do basal tests, you might want to keep track of where you are at in your hormone cycle at the time of the test. Experts say to just not test basal rates when you are restarting your cycle, so to speak. I find that completely wrong in my own body. When I basal tested with Integrated Diabetes in 2014, we accepted data from my basal tests during all different times of my cycle other than menstruation. It took me years to realize that depending on which week each rate was “set”, it could be much stronger than what I needed other weeks…so 7AM-10AM was “discovered” during an insulin resistant time of the cycle and was set relatively way too high compared to other basal rates set during insulin sensitive times of my cycle. So my whole day of basal rates was set inconsistently and lead to weird stuck highs and weird tanking lows because of this oversight/lack of discovery.

In a nutshell, I find that Week 2 of my cycle is the best time to set basal rates as there is the most consistent insulin sensitivity at that time in each cycle. And then I scale up or down by percentages every few days ranging from -10% to +25% depending on my hormones and results. No one will tell you any of this. So ultimately, keep an eye on big picture patterns as much as possible…and try not to sweat this kind of rise from last night because it could just be that you are a bit more insulin resistant today than you will be 10 days from now. And then that rise might not even show up when you are more sensitive.

There are a lot of posts about this topic in the Women’s Health Category. I’m happy to talk by private message if you ever want to. No pressure! Just throwing it out there as an offer.


Good suggestion. Instructions I’ve read across the board say dinner 4 hours before testing, but I think I’ll try to do even earlier seeing as I had a some ups and downs on account of dinner miscalculations.

Regarding testing and the cycle: I’m happy to talk about it “publicly” here as I think this info could be helpful for others!

I have read about this and heard that week 2 is the most stable time regarding insulin sensitivity! That’s where I am currently, which is why I started testing last night and want to keep testing each day/night for the next week. Once I get to testing other weeks and trends and the varying sensitivity I’ll be posting that in the Women’s Health Category :grinning:

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Perfect!! I’d love that!!

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Something else to keep in mind … some people (including me) find bolus activity times and basal response times are nowhere near the same. For me, a Humalog bolus starts dropping BG in about half an hour, but a change in basal won’t start having any effect for about 2 hours. So identify the point at which you want to stop dropping, and then reduce the basal starting 2 hours before that and see what happens. Your times will vary.

Try one change at a time. If you make all three, you won’t know which one worked, or you won’t know which change made things worse, so you won’t know what change to make next time.

That makes sense. Thanks! I wasn’t sure on the whole timing of the adjustments and such but I’ll have to test out and see.

Again makes sense and I hadn’t thought of that. Thanks!!!

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OK, update time everyone

I did another overnight basal test last night with the same settings

. This time I finished eating by 6:30 and did not exercise at night and didn’t deal with treating any lows before going into the test.

BG at 10PM was 130 and at 7AM was 64 (both read via finger stick, dex was 120 at 10PM and 74 at 7AM). **Note that for this test I woke up an hour later than the last so the time frames of the graphs differ by an hour.

I am thinking of lowering my overnight basal before testing again because I don’t like the thought of running on the lower side at night (for my sleep mainly).

Maybe I’ll do:
|8p-9p: 1|
|9p-12a: .975|
|12a-2a: 0.85|
|2a-3a: 0.85|
|3a-6a: 0.9|

And then I’ll see how that goes with another test?


I would be more comfortable with an overnight basal rate that results in a BG higher than desired.

And then work to slowly bring it down.

As opposed to the opposite (overnight BG lower than desired and slowly trying to bring it up).

With an overnight decreasing BG with no suspend technology in place, I would hold off on trying to fine-tune the basal and take a step back. Modify the basal to get the BG bumped back up then go back to work trying to fine tune.
If this was me, I would decrease basal rates by 20% starting at 10PM:
10:00 PM - 1.0 ==> 0.8 u/hr
12:00 AM - 0.85 ==> 0.68 u/hr
3:00 AM - 0.9 ==> 0.72 u/hr

Verify that has either level or increasing overnight BG and then continue work on fine tuning to bring it down into level.

But that is me working with someone who is dependent on me. I assume the considerations are different when it is YOU working on YOU. Without being able to be in anybody else’s head, I would tend to assume I am more risk averse.

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Oop let me clarify. I use suspend ON low with my medtronic 670g responding to my guardian sensor. My low measure is at 55; I have tried the suspend BEFORE low (with overnights and daytimes) but I found that it would result in me going higher than I would like, correcting too aggressively then diving down again, and the cycle goes on…

Does that change your view of recommending basals decrease by 20%?

Additionally, I liked @Beacher 's recommendation of only changing one basal period at a time to see how that affects my #s since I am going to basal test the other periods of the day after I’m happy with the overnights. That being said, the 8/9 to midnight is where you would lower first?

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