Basal Insulin Types

Every basal here is dosed correctly to hold blood sugar steady — yet the shape of its curve alone still moves it. NPH is the outlier.

Basal (background) insulin covers the glucose your liver releases all day, holding blood sugar steady between meals and overnight. A perfectly flat basal is the goal — and the flatter a real insulin is, the fewer surprises it brings. This simulator doses each one at a rate that is correct on average, so any blood-sugar movement you see comes only from the shape of the curve, not from too much or too little insulin.

There's nothing to change but the type — no dose, no timing — because the curve itself is the whole point.

Pay attention to:

How glargine and especially degludec sit almost flat, holding blood sugar steady all day
How NPH has a real peak in the middle of its cycle that drives blood sugar down — the classic low that often needs a snack
How NPH fades before 24 hours, leaving a gap where blood sugar can drift up
Why moving between a flat long-acting insulin and NPH is not a simple like-for-like swap

For educational purposes only. These curves are approximations of published insulin profiles — not dosing advice. Never change a basal insulin or dose without your care team.

What's happening here?

The green curve is the insulin working in your body over a day, at steady state (dosing at the same time each day). It's drawn downward because insulin's job is to pull blood sugar down — the deeper the curve, the harder it's pulling. The blue line is blood sugar. Because the dose is correct on average, a perfectly flat pull would hold blood sugar perfectly flat. Where the pull deepens — more insulin than your body needs at that moment — blood sugar falls; where it eases off, blood sugar drifts back up.

Glargine (Lantus, Basaglar, Toujeo) is essentially peakless over ~24 hours, and degludec (Tresiba) is flatter still and lasts ~42 hours — its overlapping doses smooth into a nearly constant background. Detemir (Levemir) is flatter than NPH but has a gentle peak and is dose-dependent, so it's often split into two daily doses.

NPH is the outlier — and the reason switching matters. NPH is intermediate-acting, not long-acting, even though it's a once- or twice-daily background dose. It has a pronounced peak around 5-7 hours, and blood sugar keeps dropping after it, so the low often lands mid-afternoon or overnight — even at the perfectly correct dose. Someone moving from glargine or degludec (flat) to NPH — for cost, access, or a hospital formulary — can be caught off guard by a low they've never had, right when NPH peaks, because nothing about a flat insulin prepared them for it. NPH also fades before the next dose, leaving a gap. Knowing the peak is coming lets you plan a snack for it, or ask your care team about splitting or timing the dose.

Applying this to your own life

Basal insulins are not interchangeable — a "correct" daily dose of NPH and of degludec behave completely differently, and they don't last the same length of time. Getting steady coverage all day sometimes means splitting the dose or adjusting when you take it. To stay ahead of it:

Know whether your basal peaks. Flat insulins (glargine, degludec) rarely cause peak-driven lows; NPH does, on a predictable daily schedule.
Plan for NPH's peak. If you take NPH, a snack timed to the 5-7 hour peak — or a dose split arranged with your care team — can prevent the classic mid-cycle low.
Be extra careful when switching. Moving to or from NPH changes your low-risk pattern; monitor closely for the first days and ask your care team how to adjust.
Match the schedule to the duration. A shorter-acting basal (NPH, and sometimes detemir) fades before 24 hours — splitting it into two daily doses keeps background insulin going all day, instead of leaving a gap where blood sugar drifts up.