[Tarana]

If it's a reader you use pretty much daily, use the Sleep Mode. If it's a device you use only occasionally (like a backup), you are best off turning it off (but turning it on and using it for a day every few months). Letting it drain the battery shortens the life of the battery. Even leaving it off for long periods without usage drains the battery.

For comparison, my brother and I bought Kindle Keyboards the same day. After a year, he shut his off and didn't use it for 2 years. He gave it to me. My Kindle was used nearly daily. When I received his kindle, it held a charge half as long as mine that had been in daily use.