OS X El Capitan: If the sleep indicator light flashes
If your computer has a sleep indicator light, it’s normal for it to slowly pulse (brighten and dim) when your computer is in sleep. If the sleep indicator light flashes (turns completely on and off repeatedly), the computer may need service.
If you hear beeps during startup
If you hear beeps when your Mac starts up instead of the normal startup tone, and the sleep indicator light flashes, your Mac might need service.
If the sleep indicator light flashes during startup
If you don’t hear beeps, but the sleep indicator light flashes five times when you start up your Mac or wake it from sleep,
You can continue using your Mac before it’s serviced. Use a different battery, or connect the power adapter and plug it into a power source.