sleep formula

have you ever noticed how two extended periods of sleep, when added together do not equal the sum total amount of rest garnered from one contiguous sleep? i had to get up this morning to run an errand and thus didn't get my one day per week of sleeping with no alarm until i wake up. therefore, when i got home i took a nap which ended up going on and on for about 3-3.5 hours. however, if that amount of time was added to the amount of time i slept the nite before, it sure doesn't equal the amount of sleep i felt i had had by this evening. know what i mean?