How It's Measured. Measurement of BMR is more restrictive than RMR. Measuring BMR requires a clinical procedure that takes place in a hospital or laboratory and requires the subject to spend the night. BMR is taken first thing in the morning, after eight hours of sleep and a 12 hour fast while the subject is lying down.RMR does not require a sleepover, and is measured at any time of day following a three to four hour fast.MR is taken first thing in the morning, after eight hours of sleep and a 12 hour fast while the subject is lying down. RMR does not require a sleepover, and is measured at any time of day following a three to four hour fast.