Basal Insulin

TL;DR

Basal insulin’s job is to cover the glucose your liver releases all day and night. You can deliver basal two ways: long-acting injections (flat profile) or pump basal (variable hourly rates). If basal is close to right, your glucose overnight stays roughly flat without food or bolus insulin in the system.

Simple

Watch the video or read on:

Deep

Basal insulin comes in two forms. Both have the same purpose: to move the glucose released by your liver into cells to create energy. Liver glucose output is not flat — it changes across the day.

Basal by injection (long-acting insulin)

The first type is long-acting insulin by injection. These are designed to work steadily over 24 hours. Examples include Lantus, Levemir, degludec (Tresiba), and others. Before I went onto a pump, I used 12 units of Levemir, which averaged roughly 0.5 units per hour.

The picture above shows why injections can mismatch liver glucose output. My glucose sometimes rose overnight and dropped during the day because my basal profile didn’t flex with the liver. That mismatch is one big reason people move to pumps.

This also explains why people with type 1 diabetes who retain some insulin production often find it easier to stay stable overnight.

Basal by pump (fast-acting insulin)

The second type of basal is delivered by a pump using fast-acting insulin. The pump delivers an hourly basal rate that can be adjusted to match liver output.

Fast-acting insulins used in pumps include NovoRapid, Humalog, Apidra, Fiasp, and Lyumjev. I use Fiasp in a Medtronic VEO pump and run five time-blocks to better match my liver. Total basal is still ~18 units, but delivered like this:

00:00–03:00 = 0.75 units/hour
03:00–09:00 = 0.80 units/hour
09:00–14:00 = 0.50 units/hour
14:00–20:00 = 0.70 units/hour
20:00–24:00 = 0.75 units/hour

I’ve had fewer highs and lows overnight since pumping. Being attached to a little machine becomes tolerable… but only just.

Grace and Jude, a pump might not suit you. If not, that’s fine — there are ways to reduce mismatch with injections. We could split basal into two doses (for example, 60% before bed and 40% on waking). Expect some trial and error and periodic tinkering.

How do I know if basal insulin is set correctly?

A simple basal check: have your last meal at about 18:00 and watch what happens overnight. By ~23:00 your fast-acting bolus insulin should be out of your system.

If glucose does not rise or fall more than ~2.0 mmol/L (36 mg/dL) overnight, basal is close.
If glucose rises more than ~2.0 mmol/L (36 mg/dL), basal likely needs increasing by ~10–20%.
If glucose drops more than ~2.0 mmol/L (36 mg/dL), basal likely needs reducing by ~10–20%.

Reality check: liver output changes night-to-night depending on the day’s inputs. For example:

Liver glucose output is often higher after a very high-fat evening meal — pizza taught me this.
Liver glucose output is often lower on exercise days — more on this in Exercise.
Liver glucose output can be minimal after heavy alcohol. I learned a lot about this between 2008 and 2017 — see Partying with T1D.

This is why a CGM with alarms is your guardian angel overnight (even if it sometimes annoys you by waking you up).

Why AID systems dominate overnight

The best way to manage changing liver output is to change basal as glucose changes. That’s exactly what automated insulin delivery (AID) systems do: they track glucose trends and speed up or slow down basal accordingly.

AID systems are the bee’s knees overnight. I benefited immediately when I trialled DIY looping, Tandem t:slim X2 with Control-IQ, Omnipod 5, and the MiniMed 780G. This graphic explains the overnight issue and how one AID system, CamAPS FX, dominates.

I created an AID system guide for those who want to go deeper.

Why “dominant overnight” doesn’t always mean “dominant all day”

AID systems react to glucose by adjusting basal. That works brilliantly when glucose is moving slowly and there’s no food, bolus insulin, exercise, or stress in play. When is glucose slow and clean like that?

Overnight.

During the day, a basal reduction is too slow to stop a fast drop after a meal where bolus insulin was too high. The reduction takes ages to bite, so glucose keeps falling unless you treat the hypo.

Then glucose often rebounds fast after hypo treatment because the temp basal decrease has left you short of circulating insulin — a high-glucose hangover.

This is shutting the stable door after the horse has bolted.

Remember this: insulin is a slow mover of glucose. (See: fast and slow movers.)

During the day, you need tools that work in ~20 minutes, not 2–3 hours.

The same logic applies to rising glucose after a meal where insulin was too little: a basal increase takes too long to prevent the peak.

Practical

Use overnight fasting checks to sense-check basal, but expect variability. Don’t “fix” basal from one weird night — look for repeated patterns, ideally with CGM data.

References

What’s next

Next step: bolus insulin.