Water will travel across ceilings and then escape from any point. I don't want to sound alarmist but don't be turning on that light as water and electricity can make a bad combination. I think you would be best to turn off the water supply to your house usually found under the sink in the kitchen and get up into the attic as soon as possible to have a look. You most probably have a leak and it will need fixing. Speed is usually best in these cases so as to limit damage.
I'd probably turn on the taps upstair and down to drain as much water as quickly as possible, remembering to store some so as to have a supply of water for later.
Best of luck