What they want is concentrated urine, the most concentrated possible so the hCG will show up in enough quantity in the urine to be measurable. If you got up at 5 am and drank a quart of water, then took a pregnancy test at 7 am, it would not give you a good result because your urine would be too dilute. By the same token, if you had been up all day but did not eat or pee from noon to 8 pm for whatever reason, and then took a test, it would give you an accurate result. The first-morning urine instruction is to try to get concentrated urine, since so many women test as early as possible. (Later in pregnancy you can pee on a stick at any time of day and get a positive.)
if u read the directions it will tell you if it's mandetory to use the morning urine, most say that its' not mandetory but just that it's recommended because it contains the highest level of the pregnancy hormone (hCG *human chrorionic gonadotropin)
An hpt is accurate at any time of day but is most accurate with first morning urine.