Just add a channel to the light intensity and put two keys, one for the lowest intensity, which I presume is 0, and one for the highest, a few frames after the first, depending on how gradual you want the light up to be. If you want it to stay off for longer, just add the low key after the default frame 0 key and change the tension to 1 so the graph is flat.
after tweaking it, chose repeat as Post Behaviour and there you have it.
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