Why Do My Blood Sugar Levels Drop During Sleep?

Shana Spindler PhD
Shana Spindler PhD
July 22, 2021

For our first article in the “You Ask, We Answer” series, let’s tackle a frequently asked question from the Sync inbox: Why do my blood sugar levels drop during sleep?

This is a great question. For now, we’ll focus on overnight glucose levels in the non-diabetic population. Check back for a future post about nighttime glucose monitoring in individuals with diabetes. 

Normal overnight blood sugar levels

We’re just beginning to learn about overnight glucose regulation in healthy, non-diabetic people. Many continuous glucose monitoring (CGM) studies focus on individuals with diabetes. For the diabetic population, glucose readings below 70 mg/dL (3.9 mmol/L) during sleep are considered too low, a condition that doctors term nocturnal hypoglycemia. But for healthy, non-diabetic individuals, the critical number to reach hypoglycemia is much lower at 55 mg/dL (3.1 mmol/L) or less.

A 2019 CGM study of healthy individuals supports the idea that occasional overnight glucose readings as low as 55 mg/dL (3.1 mmol/L) are part of normal glucose regulation.

A team of physicians and scientists from 12 clinical centers around the United States collected CGM data from 153 healthy participants ranging from 7 to 80 years of age. The researchers recorded glucose readings, day and night, for an average of eight days. Their results show a fairly consistent regulation of blood glucose during sleep, with 99% of the glucose readings falling within the 70 to 140 mg/dL (3.9 to 7.8 mmol/L) range between midnight and 6 a.m.

“Essentially, average glucose readings were similar during daytime and nighttime” says Dr. Viral Shah, associate professor at the University of Colorado Medicine, physician, and lead author of the study. “This was expected, as we anticipated these healthy participants would have normal pancreatic insulin secretion and normal physiological interplay between different hormones, such as insulin and glucagon.”

In their study, the average overnight glucose reading of 98 mg/dL (5.4 mmol/L) was only slightly lower than the daytime average of 100 mg/dL (5.6 mmol/L), and most participants experienced some glucose levels of 55 to 69 mg/dL (3.1 to 3.8 mmol/L). According to the study authors, their findings support the recommendation that CGM values less than 55 mg/dL (3.1 mmol/L) are an appropriate cut-off to define clinically important hypoglycemia.

“I would not call some of the low glucose levels [between 55-69 mg/dL (3.1-3.8 mmol/L)] in our study hypoglycemic dips, as these glucose levels are considered normal in the non-diabetic population,” Dr. Shah explains.  

What causes blood sugar to dip during sleep in non-diabetic individuals?

The 2019 study suggests that overnight drops below 70 mg/dL (3.9 mmol/L) on a glucose monitor can occur in healthy individuals, but why the dips happen is still unknown.  

“I don’t know of any body of literature explaining why this occurs,” says study co-author Dr. Jennifer Sherr, physician and associate professor at the Yale School of Medicine. “What we’ve been able to do with more accurate sensors is to better define what normal is. People can do absolutely fine in that 60 to 70 mg/dL (3.3 to 3.9 mmol/L) range.”

In an older 2009 sleep study of 13 individuals with normal glucose tolerance, researchers found that CGM measurements during REM sleep were slightly, but significantly, lower than non-REM sleep. However, the study was small, and CGM technology has improved greatly over the past decade. Additional research is warranted to confirm the impact of REM sleep on glucose levels.

While studies are now showing it’s normal for blood sugar to drop slightly during sleep, it’s important to understand that this is a tightly controlled process. For example, a 2021 CGM study on blood sugar levels in healthy, non-diabetic individuals found that overnight glucose levels remained similar after a day with exercise versus a day without exercise. Surprisingly, exercise type (aerobic versus resistance training) didn’t show a significant difference in overnight average glucose readings. These results held true for both men and women in the study, and whether or not the participants exercised in the morning, afternoon, or evening.

The same study also looked at the effect of alcohol consumption on glucose levels during sleep in non-diabetic individuals. While an overnight drop in blood sugar after drinking alcohol is well known in individuals with diabetes, the researchers did not see a change in overnight glucose following alcohol consumption in their non-diabetic study participants.

Ultimately, the physiological pathways that control nighttime blood glucose levels in healthy individuals are neither fully mapped out nor well-understood. But with the evolving technology of CGM, and the participation of healthy volunteer study participants, researchers anticipate being able to tease out the biological processes that control overnight blood sugar regulation.

When should I be concerned about low overnight blood sugar?

Individuals with diabetes must follow blood sugar levels closely, being careful not to dip too low into nocturnal hypoglycemia. But what about individuals without diabetes? Symptoms of non-diabetic hypoglycemia are similar to those for the diabetic population. Non-diabetic hypoglycemia symptoms include hunger, shakiness, anxiety, sleepiness, dizziness, confusion, sweating, irritability, weakness, and sometimes trouble speaking.

“For non-diabetic healthy individuals, contact a doctor if you’re having recurrent glucose levels below 55 mg/dL (3.1 mmol/L), accompanied by symptoms of hypoglycemia,” Dr. Shah says. “You should not be concerned about occasional glucose levels around 70 mg/dL (3.9 mmol/L) or slightly below 70 mg/dL (3.9 mmol/L).”

“It’s important to have someone help you understand if it’s a low reading because the glucose monitor is being compressed when you roll over during sleep,” Dr. Sherr adds. “If you’re symptomatic with low glucose and you see repeated patterns emerging over several days, it’s worth discussing with your doctor to see if there’s an underlying problem.” 

References 

For our first article in the “You Ask, We Answer” series, let’s tackle a frequently asked question from the Sync inbox: Why do my blood sugar levels drop during sleep?

This is a great question. For now, we’ll focus on overnight glucose levels in the non-diabetic population. Check back for a future post about nighttime glucose monitoring in individuals with diabetes. 

Normal overnight blood sugar levels

We’re just beginning to learn about overnight glucose regulation in healthy, non-diabetic people. Many continuous glucose monitoring (CGM) studies focus on individuals with diabetes. For the diabetic population, glucose readings below 70 mg/dL (3.9 mmol/L) during sleep are considered too low, a condition that doctors term nocturnal hypoglycemia. But for healthy, non-diabetic individuals, the critical number to reach hypoglycemia is much lower at 55 mg/dL (3.1 mmol/L) or less.

A 2019 CGM study of healthy individuals supports the idea that occasional overnight glucose readings as low as 55 mg/dL (3.1 mmol/L) are part of normal glucose regulation.

A team of physicians and scientists from 12 clinical centers around the United States collected CGM data from 153 healthy participants ranging from 7 to 80 years of age. The researchers recorded glucose readings, day and night, for an average of eight days. Their results show a fairly consistent regulation of blood glucose during sleep, with 99% of the glucose readings falling within the 70 to 140 mg/dL (3.9 to 7.8 mmol/L) range between midnight and 6 a.m.

“Essentially, average glucose readings were similar during daytime and nighttime” says Dr. Viral Shah, associate professor at the University of Colorado Medicine, physician, and lead author of the study. “This was expected, as we anticipated these healthy participants would have normal pancreatic insulin secretion and normal physiological interplay between different hormones, such as insulin and glucagon.”

In their study, the average overnight glucose reading of 98 mg/dL (5.4 mmol/L) was only slightly lower than the daytime average of 100 mg/dL (5.6 mmol/L), and most participants experienced some glucose levels of 55 to 69 mg/dL (3.1 to 3.8 mmol/L). According to the study authors, their findings support the recommendation that CGM values less than 55 mg/dL (3.1 mmol/L) are an appropriate cut-off to define clinically important hypoglycemia.

“I would not call some of the low glucose levels [between 55-69 mg/dL (3.1-3.8 mmol/L)] in our study hypoglycemic dips, as these glucose levels are considered normal in the non-diabetic population,” Dr. Shah explains.  

What causes blood sugar to dip during sleep in non-diabetic individuals?

The 2019 study suggests that overnight drops below 70 mg/dL (3.9 mmol/L) on a glucose monitor can occur in healthy individuals, but why the dips happen is still unknown.  

“I don’t know of any body of literature explaining why this occurs,” says study co-author Dr. Jennifer Sherr, physician and associate professor at the Yale School of Medicine. “What we’ve been able to do with more accurate sensors is to better define what normal is. People can do absolutely fine in that 60 to 70 mg/dL (3.3 to 3.9 mmol/L) range.”

In an older 2009 sleep study of 13 individuals with normal glucose tolerance, researchers found that CGM measurements during REM sleep were slightly, but significantly, lower than non-REM sleep. However, the study was small, and CGM technology has improved greatly over the past decade. Additional research is warranted to confirm the impact of REM sleep on glucose levels.

While studies are now showing it’s normal for blood sugar to drop slightly during sleep, it’s important to understand that this is a tightly controlled process. For example, a 2021 CGM study on blood sugar levels in healthy, non-diabetic individuals found that overnight glucose levels remained similar after a day with exercise versus a day without exercise. Surprisingly, exercise type (aerobic versus resistance training) didn’t show a significant difference in overnight average glucose readings. These results held true for both men and women in the study, and whether or not the participants exercised in the morning, afternoon, or evening.

The same study also looked at the effect of alcohol consumption on glucose levels during sleep in non-diabetic individuals. While an overnight drop in blood sugar after drinking alcohol is well known in individuals with diabetes, the researchers did not see a change in overnight glucose following alcohol consumption in their non-diabetic study participants.

Ultimately, the physiological pathways that control nighttime blood glucose levels in healthy individuals are neither fully mapped out nor well-understood. But with the evolving technology of CGM, and the participation of healthy volunteer study participants, researchers anticipate being able to tease out the biological processes that control overnight blood sugar regulation.

When should I be concerned about low overnight blood sugar?

Individuals with diabetes must follow blood sugar levels closely, being careful not to dip too low into nocturnal hypoglycemia. But what about individuals without diabetes? Symptoms of non-diabetic hypoglycemia are similar to those for the diabetic population. Non-diabetic hypoglycemia symptoms include hunger, shakiness, anxiety, sleepiness, dizziness, confusion, sweating, irritability, weakness, and sometimes trouble speaking.

“For non-diabetic healthy individuals, contact a doctor if you’re having recurrent glucose levels below 55 mg/dL (3.1 mmol/L), accompanied by symptoms of hypoglycemia,” Dr. Shah says. “You should not be concerned about occasional glucose levels around 70 mg/dL (3.9 mmol/L) or slightly below 70 mg/dL (3.9 mmol/L).”

“It’s important to have someone help you understand if it’s a low reading because the glucose monitor is being compressed when you roll over during sleep,” Dr. Sherr adds. “If you’re symptomatic with low glucose and you see repeated patterns emerging over several days, it’s worth discussing with your doctor to see if there’s an underlying problem.” 

References