Putting your laptop to sleep does 2 things:
1. All devices except the CPU shuts down.
2. The CPU consumes MUCH less power than it usually does to keep the data on RAM. So when you turn your computer back up, you can quickly resume back to work, since the CPU was still running, meaning the memory/processes were left intact. The Hard Drive spins very, very slowly or just completely shuts off.
You're not really "wasting" energy if you're going to return to your computer soon. If you put your computer to sleep for, say, a day, then the energy to keep your CPU on was wasted. You should have just turned off the laptop.
Hibernating is a different thing. Hibernate writes all the contents of the RAM onto the Hard Drive, then completely shuts down. CPU and all. Yes, the battery will lose energy, but hey, every battery very very slowly loses energy over time. Just the way it goes. When you turn back on the computer, it reloads the content from hard drive back to the RAM, which is a faster process than turning off the computer and re-turning it on. However, because the computer has to turn back on and read the hard drive (which is slow), hibernate is much slower than sleep.