Overview
Basal insulin covers the glucose your liver releases all day and night. It is background insulin, working independently of food. Basal can be delivered in two ways: long-acting injections, which provide a relatively flat insulin profile, or pump basal, which allows insulin delivery to vary hour by hour.
If basal insulin is close to right, glucose overnight tends to stay roughly flat when there is no food or bolus insulin in the system. When basal is mismatched, glucose may drift up or down in the background, making everything else harder to manage.
This is why basal insulin sits early in the Foundations sequence. If basal is off, bolus insulin feels unpredictable, corrections feel random, and diabetes management can start to feel personal when it isn’t.
The detail
Your liver continuously releases glucose into the bloodstream. That output is not flat. It changes across the day and night in response to hormones, sleep, stress, illness, exercise, alcohol, and earlier food intake.
Basal insulin’s role is to move this liver-derived glucose into cells to be used for energy. It is not designed to cover meals, and it cannot respond quickly to rapid glucose changes.
The graphic above shows the core basal challenge: liver glucose output rises and falls across the day, while insulin delivery—particularly with injections—is often relatively fixed.
Basal by injection (long-acting insulin)
Long-acting basal insulin is designed to work steadily over many hours. Examples include Lantus, Levemir, and degludec (Tresiba). The aim is to provide a stable background insulin level.
The limitation is that liver glucose output is not stable. As a result, glucose may rise at some times of day and fall at others, even when the total daily basal dose is reasonable. This mismatch is one of the main reasons people struggle with unexplained overnight drift or daytime dips.
This also helps explain why people who retain some endogenous insulin production often find overnight stability easier: even small amounts of internal insulin can buffer basal mismatch.
Basal by pump (fast-acting insulin)
With insulin pumps, basal insulin is delivered using fast-acting insulin in tiny amounts throughout the day and night. The defining feature is flexibility: pump basal can be adjusted hour by hour to better match changes in liver glucose output.
Fast-acting insulins used in pumps include NovoRapid, Humalog, Apidra, Fiasp, and Lyumjev. In practice, pump users often use multiple time blocks across 24 hours. The goal is not perfection, but reducing mismatch.
Illustrative example: a pump basal profile may be set higher overnight and lower during parts of the day, while keeping the total daily basal similar. This reflects physiology rather than fighting it.
Pump therapy is not suitable or desirable for everyone. Injections can work extremely well. Understanding why mismatch happens helps shift blame away from behaviour and back onto biology.
How do I know if basal insulin is set correctly?
Basal insulin is easiest to assess overnight, when food, bolus insulin, and activity are largely absent. This exposes the background insulin signal.
- Relatively stable overnight glucose suggests basal is close.
- Consistent overnight rises suggest basal may be insufficient.
- Consistent overnight drops suggest basal may be excessive.
The graphic above shows glucose remaining broadly stable overnight. This is what “close enough” basal often looks like.
Liver glucose output varies night to night depending on the day’s inputs. Single nights are not reliable signals; repeated patterns matter more.
- Liver output is often higher after very high-fat evening meals.
- Liver output is often lower on exercise days.
- Liver output can be minimal after heavy alcohol intake.
This is why a CGM with alarms is particularly valuable overnight.
Why AID systems dominate overnight
Automated insulin delivery (AID) systems adjust basal insulin in response to CGM trends. They increase or decrease basal as glucose changes.
Overnight, when glucose moves slowly and there is no food, bolus insulin, exercise, or stress in play, AID systems consistently outperform fixed basal delivery. This is not because they are clever, but because basal needs are not static.
Why “dominant overnight” doesn’t mean “dominant all day”
AID systems adjust basal, which is a slow mover of glucose. This works well overnight, but during the day basal changes are often too slow to prevent rapid post-meal rises or drops.
This is why insulin is described as a slow mover of glucose. During the day, faster tools are often needed.
What’s next
Next step: Bolus insulin.
Foundations / related content
The order shown below is recommended, but navigate as you see fit.
- What is Diabetes?
- Continuous glucose monitoring (CGM)
- Basal insulin
- Bolus insulin
- Correction insulin
- Carbohydrate counting
- Three balanced meals
- Hypoglycaemia
- Measuring success
