Why Your Morning Blood Sugar Runs High

Many people with type 2 diabetes find that their fasting reading, taken before breakfast, is higher than their reading at bedtime, even though they ate nothing overnight. This article explains the usual causes and how you and your doctor can work out which one applies to you.

The dawn phenomenon

In the early morning hours, usually between about 3 a.m. and 8 a.m., the body releases hormones that prepare it to wake up, including cortisol, growth hormone and glucagon. These hormones signal the liver to release stored glucose into the blood. In people without diabetes, the pancreas releases extra insulin to balance it. In type 2 diabetes, the body cannot respond fully, so blood sugar climbs before you get out of bed.

The dawn phenomenon is common and is the most frequent reason for a high fasting number when bedtime readings look fine.

The Somogyi effect

A less common cause is a rebound after a low. If blood sugar drops too far during the night, often because of insulin or a sulfonylurea taken in the evening, the body releases hormones to raise it, and the reading can overshoot by morning. Researchers now think this pattern is less frequent than once believed, but it matters because the fix is the opposite of the fix for the dawn phenomenon. Signs include night sweats, nightmares or a headache on waking.

Other causes

• An evening snack or a late dinner, especially one high in carbohydrate, can still be raising blood sugar overnight.
• Medication that wears off before morning, or a dose that is too low, can leave the early hours uncovered.
• Short or poor sleep, including untreated sleep apnea, raises stress hormones and insulin resistance.
• Illness or infection raises blood sugar around the clock.

How to tell which one it is

The most direct way is to see what happens during the night. Check your blood sugar at bedtime, again around 2 or 3 a.m., and when you wake, on two or three nights. A continuous glucose monitor gives the same picture without the alarm clock.

• If the 3 a.m. reading is normal or high and the morning reading is higher, the pattern points to the dawn phenomenon.
• If the 3 a.m. reading is low and the morning reading is high, the pattern points to a rebound, and your doctor will likely want to lower an evening dose.
• If the bedtime reading is already high, the cause may be the evening meal or snack.

What may help

Share your overnight readings with your doctor before changing any medication. Depending on the pattern, they may change the timing or type of medication, or switch to a longer-acting form. Some people find that eating dinner earlier, keeping the evening meal lower in carbohydrate and taking a short walk after dinner lowers their morning number. Others see a difference from a consistent sleep schedule.

The American Diabetes Association suggests a fasting and before-meal range of 80 to 130 mg/dL for many adults with diabetes, though your doctor may set a different target.