This is Dr. David. I am reviewing your question now and will be with you momentarily.
it is because of gravity pulling fluid like lymphatic fluid down into your legs during the day when you are up or sitting up.
at night time, you lay flat and your lymphatic fluid is pushed out of your leg
elevating your legs during the day, wearing compression stockings, exercise and walking during the day can help decrease swelling in that spot of your leg.
let me know if you have questions.