Morning highs can be baffling. After all, you just spent the past nine hours or so sleeping—in other words, not ingesting any carbs. What’s going on?

What causes high morning blood glucose?

Two main culprits prompt morning highs: the dawn phenomenon and waning insulin. A third, much rarer cause, known as the Somogyi effect, may also be to blame.

The occasional morning high will have little impact on your A1C, a measure of your average blood glucose (blood sugar) levels over time that indicates how well managed your diabetes is. But if those highs become consistent, they could push your A1C up into dangerous territory.

The dawn phenomenon

In the early hours of the morning, hormones, including cortisol and growth hormone, signal the liver to boost the production of glucose, which provides energy that helps you wake up. This triggers beta cells in the pancreas to release insulin in order to keep blood glucose levels in check. But if you have diabetes, you may not make enough insulin or may be too insulin resistant to counter the increase in blood glucose. As a result, your levels may be elevated when you wake up. The dawn phenomenon does not discriminate between types of diabetes. Approximately half of those with either type 1 or type 2 experience it.

Waning insulin

If your insulin level falls too low overnight, your blood glucose rises. The reasons for the drop in insulin vary from person to person, but it most commonly occurs when your insulin pump settings provide too little basal (background) insulin overnight or if your long-acting insulin dose is too low. Insulin duration—how long the drug works in your body—also comes into play. If you inject your long-acting insulin early, it may not last into the morning.

The Somogyi effect

Named after Michael Somogyi, PhD, a chemist who was the first to describe it in the 1930s, the Somogyi effect is the body’s response to low blood glucose (hypoglycemia) during the night. Say you miss dinner or take too much insulin after your evening meal. Your blood glucose may fall too low overnight. Your body makes more glucose in order to compensate, and you wake up with high blood glucose.

So, what can you do?

Gather the clues

If a pattern of frequent morning highs emerges during your routine glucose monitoring, check your blood glucose levels at bedtime, in the middle of the night and first thing when you wake up to develop a better understanding of your glucose patterns. If you use a continuous glucose monitor (CGM), you can sleep through the night and it’ll gather the data you need. If you don’t use one, see if your doctor can provide a temporary loaner.

Identify the culprit

Your readings will tell you and your doctor when your highs and lows occur and that, in turn, will help narrow the cause of the problem.

If the data shows you’re high at bedtime, the culprits are likely food and medication.

If you have high blood glucose before you go to sleep, the elevated level can persist until morning. A large dinner or a snack at bedtime can cause elevated blood glucose levels that last all night, as can too low a dose of insulin with your evening meal. Adjusting your medication or what and when you eat may help.

If the data shows you’re in range at bedtime, the culprit is likely too little medication.

You may go to bed with blood gluocse levels within your target range, but that doesn’t mean they’ll stay that way overnight. For example, if you are taking a long-acting insulin in the morning and it wears off before the next dose the following day, you would see morning high blood glucose. Changing the timing of your long-acting insulin injection, or switching to a twice-daily basal insulin or an ultra-long-acting insulin, might fix the problem.

If the data shows you’re high in the wee hours, the culprit is likely dawn phenomenon.

If you’re experiencing the dawn phenomenon, which raises your blood glucose between approximately 3 and 8 a.m., your doctor may recommend that you avoid increasing your long-acting insulin. While a higher dose of insulin will bring your morning highs down to normal, it could cause too great a drop in your blood glucose after you first go to sleep, but before your blood glucose starts to rise in the early hours of the morning. Sometimes the only way to adequately address the dawn phenomenon is with an insulin pump, which you can program to automatically deliver more insulin in the early morning hours.

If you don’t use insulin, it may take a good bit of trial and error before you and your health care provider figure out the best medication and lifestyle strategy to help reduce morning highs.

Work it out

Exercise can also help you manage your morning highs. If you have waning insulin, an after-dinner walk or other workout can help keep your blood glucose down overnight. But use caution when exercising before bedtime. The blood glucose–lowering effects of exercise can last for hours, so if you work out before bed, you risk going low overnight.

Morning exercise may be best if your blood glucose data has shown a trend of nighttime lows after late afternoon or evening exercise. Working up a sweat in the a.m. is a good idea for anyone experiencing the dawn phenomenon, too—it can help burn up that extra blood glucose.

Try, try again

There’s no single recipe to control morning highs. What works for one person may not work for you. It may take time to find the best strategy to keep your blood glucose at the right level in the morning while avoiding hypoglycemia overnight. In rare cases, the ideal balance can’t be found. For those people, their doctors might change their morning blood glucose goal so that it’s a little bit higher, as long it stays within goal the rest of the day. But most people will be able to figure out what’s happening and what to readjust.