In order to conserve power and extend battery life in laptops, Apple offers an option to automatically spin down hard disks when not in use. This option can be found in the Energy Saver and can be configured for different energy schemes (battery or power adapter), but is limited to having the system put the hard disk to sleep whenever possible, without many configuration options. By default this will put disks to sleep that have not been accessed for 10 minutes, and while this may work for most people, others may wish to fine-tune this behavior.
While hard drives will use energy to keep the platters active and spinning, they will use the most energy per unit time when the platters spin up. This is exactly like a car, where its gas mileage will stay at moderately higher levels when cruising at steady speeds, but will drop when accelerating.
To restate that in terms of power usage, a hard drive uses the least power when in sleep or standby mode, but like a car will use the most when starting up and accelerating the disk platters to speed. Therefore even if you purposefully sleep the drive after each read session, if you frequently access the drive you will be endlessly powering it up and down, which will use more battery life, not less.
It is for this reason that Apple sets a 10-minute wait period for hard disks, because even though secondary data drives may be infrequently accessed, boot drives may be accessed regularly, and to have them continually spin down and back up not only uses more power but can cause stalls while the system waits for data from the drive.
While it is good to keep the default 10-minute wait period, there may be special circumstances in which you would want to change this period. One of these that is becoming more common is if your system has a solid-state boot disk coupled with a standard data drive. Apple has been offering this as an internal configuration for Mac Pro and iMac systems for a while, but even on laptops you can upgrade to a solid-state drive (SSD) and use external mechanical data drives. In these configurations, having an infrequently accessed data drive continually spinning may draw extra power.
With all this in mind, you can tailor your drive's spin-down time by adjusting when it goes to sleep. To do this, you will need to have the XCode developer tools for OS X installed; these can be purchased from the Mac App Store if you do not already have them. The tools come with a variety of hardware utilities, including a utility called SpindownHD. With this utility you can see the status of your disks, as well as an option for setting the idle time after which the drive will go to sleep.
You can increase this time to maintain drives in a ready state for longer, or you can decrease it down to a minute to sleep the drives sooner. Optionally, if you always want the maximum performance from your drives and never want them to be in a standby state, you can disable this feature (which also can be done by unchecking it in the Energy Saver system preferences).