A good thing to check is to see what the system thinks the battery capacity is on your machine and your friends. To do this, go to the Apple menu, About this Mac. Then click on More Info, and pick the power category on the left side. The info you want is under two sections, and using my MacBook Pro with a new battery as reference, here is what mine says:

Charge Information:
Charge remaining (mAh): 2794
Fully charged: No
Charging: No
Full charge capacity (mAh): 5484

Health Information:
Cycle count: 5
Battery health: Good

Full charge capacity is going to be the key number you want to compare, then also compare charge remaining after your sleep experiment. Verify what Bruno said about Bluetooth settings, and also ensure any USB devices are disconnected to get a good and equal measurement. Make sure the cycle count at least has a few cycles as well, as sometimes it takes a machine a bit to properly calibrate the percentage displays. I've seen my Mac hit 0%, then recalculate and pop up to 15-20% with a new battery the first time through a charge cycle.