Many people feel so tired after eating cake or cookies they have to lie down. That's because sugar wreaks havoc on your metabolism, quickly raising your blood glucose (sugar) levels and, in turn, forcing your pancreas to release insulin. The insulin then lowers your blood sugar levels quickly, which makes you feel tired.
Anything that tastes sweet has either sugar (a type of carbohydrate) or synthetic sweeteners in it. These substances interact with taste receptors on your tongue, communicating the sweetness to your brain.
If you consistently get tired after eating high-carbohydrate foods--not just sweets but also pasta, potatoes and other foods that are metabolized into glucose (sugar) by your digestive system--your body is probably having trouble keeping your blood sugar levels even.
If you get tired after eating food that has artificial sweeteners in it, your body, thinking it is about to get a lot of sugar, might be increasing your level of insulin, which makes you hungry and lowers your blood-sugar levels, making you tired.
When you eat, your body converts the food into glucose. This raises your "blood sugar" levels (the amount of glucose in your system). In response, your pancreas releases insulin to help your cells keep their glucose levels within a safe range.
Foods that greatly and rapidly raise your blood sugar levels--such as sugary sweets, white bread, pasta, corn and white potatoes--are said to be high on the "glycemic index." They make your blood sugar levels "spike." Foods that raise your blood sugar level less or more slowly, such as whole grains, vegetables and proteins are low on the glycemic index.
You can prevent spikes in your blood sugar levels by sticking to a diet consisting mainly of low-carbohydrate, low-glycemic foods such as fish, meat, fruits, vegetables and whole grains. Avoid foods with sugar and artificial sweeteners, both of which can cause quick changes in your insulin levels.