Can you measure your BMI by what you weigh when you first wake up?

Yes. Bmi is a calculated value determined by your weight in kilograms divided by your height in meters squared( kg/m2). You can use any weight throughout the day; however, your morning weight will usually be the lowest giving you the lowest calculated bmi for that day. If you're trying to use bmi to get approved for bariatric surgery, it's better to use a weight latter in the day when you're heavier.
Weight:height. The BMI is simply your weight with respect to your height - an overly simplistic and thereby essentially flawed assessment of your health. It's a bit better when used in combination with body fat measurement. That being said, a high BMI might be associated with diabetes and heart disease amongst other health conditions. You can check your weight first thing in the morning.
Yes. One can use his weight from any time during the day or night, for calculating bmi. Normal, overweight, and obese are defined by the bmi, which is a measurement that looks at one's weight in comparison to his height. Tall people are allowed to weigh more than short people are. Bmi calculators are available at the doctor's office and also on the internet. An adult with a BMI of 18.5-24.9 is normal.