Like most Hoyas, siariae prefers bright, indirect sun. If possible, it should be kept in an area where no direct sun blazes down on it's leaves.

You CAN harden it off, gradually, to full sun, but long term, it could have negative effects. I tried it with a couple of my Hoyas, and although their leaves turned a gorgeous red (instead of burning), over the winter, they suffered tremendous leaf loss. I think that full sun harms most Hoyas. Remember, in nature, they are rainforest plants (there are a few exceptions).