Hi Louise,
Bonerp beat me to it with his response. There's actually 2 things that can occur during the night that both have the effect of leaving you with a higher BS level in the morning than you went to bed with.
One of these is the Somogyi effect (named after Dr Michael Somogyi who discovered it). This is where the body senses that glucose levels are dropping to hypo levels and in response the liver pumps glucose into the bloodstream from its stored supply of glycogen. This generally results in overcompensating, leaving you with blood glucose at almost hyper levels (so is somethimes called the "bounce" or "rebound" effect). This is actually less common than many people imagine and is usually the result of having too much circulating insulin during the night)
The other one is the "Dawn Effect" where the body during the night automatically releases glucose into the blood, but as part of its normal functioning rather than in response to a specific trigger like a hypo. There are two ways in which this natural glucose release occurs:
- for about 4 hours after you have eaten your liver extracts some of the glucose in your blood and stores it as glycogen. After 4 hours (provided nothing else has been digested) the glycogen starts to be converted back into glucose in the blood. This dumping of glucose can continue for up to 12 hours, but it stops as soon as you eat something. The effect can be reduced by eating a low-carb high protein snack just before you go to bed (like a handful of nuts, a piece of cheese or ham (babybell size), or even a piece of lower carb fruit like an apricot or plum).
- during the night our bodies release hormones that help maintain and restore our body cells. These hormones (growth hormone, cortisol and catecholamines) also cause the glucose level to rise.
In both cases people with diabetes often don't have enough circulating insulin to keep this natural increase of glucose under control, so the result is a higher glucose reading in the morning.
Unfortunately in the UK the difference between the two (hypo response and normal hepatic glucose production) is not widely understood and they both tend to get lumped together under the heading "Dawn Effect", when in fact they are two very different things that just happen to have the same outcome of a high morning BG level.