A significant drop in blood sugar typically causes symptoms of hypoglycemia relatively quickly, because of the brain's dependence on a constant glucose supply. A high blood glucose level, or hyperglycemia, may or may not cause obvious symptoms.
In people with diabetes, the combination of high blood sugar and lack of insulin often leads to signs and symptoms, including: excessive thirst and hunger, unintentional weight loss, lack of energy, increased urination.
The combination of low blood sugar and/or too much insulin often leads to signs and symptoms, including: shakiness, dizziness, headache, fatigue, confusion and weakness.
Adapted from Livestrong.com (https://www.livestrong.com/article/133891-the-importance-glucose/)