Before you got diabetes, your pancreas made all your insulin. And it knew exactly when to release the insulin to the body. Like a smoke detector, your body sensed when there was more sugar in the bloodstream. It sent out the alarm—time for more insulin! Manufactured insulin comes in several types. Each type works at a different pace, and most people have to use more than one kind to mimic what their bodies used to do all by themselves.

Different types of insulin are like different types of Olympic runners— Some types of insulin are like sprinters—they start quickly, get to their top speed, and finish fast. Other types of insulin are like marathon runners—they start slower, and they keep going slow and steady for a long time. Then there are the ones in between—not as fast as a sprint and not as slow as a marathon. But no one type of insulin is better than the others. All types are important to keep your diabetes in control.

Rapid-acting insulin, often called lispro or Humalog, is the fastest insulin of all. Once you inject it, it starts to work within 15 minutes. It works hardest (or "peaks") at about an hour or so after you inject it. It’s pretty much used up in four or five hours. This kind of insulin is designed so you can inject it right before meals. It starts to work about the time you start to eat. By the time your meal is digested and sugar is beginning to move into the bloodstream, rapid-acting insulin is working the hardest at moving that sugar into the cells.

Short-acting, or "regular," insulin is also used around mealtime. It takes longer to work than rapid-acting insulin does. So you take short-acting insulin about 30 to 45 minutes before you plan to eat and it peaks at about two or three hours. It can keep working for as long as six hours. Rapid-acting and short-acting insulin are both "clear" insulins.

There are two types of intermediate-acting insulin, NPH and lente. Intermediate-acting insulin is insulin mixed with a substance that makes the body absorb the insulin more slowly. That’s why this type of insulin looks cloudy and has to be mixed before it’s injected. It takes longer to start to work, and it stays in your body for a longer time. NPH usually begins to work about two to four hours after you inject it. It peaks four to 10 hours after injection, and it keeps working for 10 to 16 hours. Lente lasts even longer. It peaks at four to 12 hours after injection and stays in the body from 12 to 18 hours. Intermediate-acting insulin works all day if you take it in the morning. A shot of intermediate-acting insulin in the evening keeps insulin in your body during the night.

Then there’s the marathon runner of the bunch, long-acting insulin, also called ultralente. It starts to work in 6 to 10 hours and can stay in the body for 20 hours or more. It has a peak, but its top speed looks a lot like its normal speed. Long-acting insulin is usually taken in the morning or before bed, like intermediate-acting insulin.

Pike’s Peak is a very steep mountain in the western United States. Visitors to Pike’s Peak take a slow train to the top of the peak, look around, and then take a train back down. Insulin works much the same way. From the time you inject insulin, the insulin level in your blood goes up, up, up—until it peaks, or reaches its highest level. Your insulin level may hang out at that highest level for a little bit. Then it starts going down, down, down—until it reaches the bottom and is out of the body altogether.

As you can see from the chart above, different types of insulin have different peaks. Rapid- and short-acting insulins zoom up to the highest peak of all, then zoom back down. It’s like taking a supersonic train to the top of Pike’s Peak. Intermediate- and long-acting insulins top out at a lower level and take longer to get up and back down. Their action is more like taking a stroll up a hill.

No matter which kind of insulin you use, it’s important to time your eating around its peaks. That’s why rapid- or short-acting insulin is usually taken before meals. When it peaks, you’ll have all the sugar from your meal in your blood.

The longer-acting insulins have an important place, too. They help keep your insulin and sugar levels steady throughout the day and night. Often, your snacks will be timed to the peaks of the intermediate-acting insulin.

If you don’t eat when your insulin peaks, you could get hypoglycemia. After all, you’d have all that insulin working like crazy and not enough sugar for it to work on! Insulin peaks are an important reason to eat on schedule—even if you don’t feel all that hungry.