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Customs checks when travelling between EU countries are on a random basis with the main purpose of finding illegal items, most notably drugs. There are no restrictions on bringing elecetronics or produce for personal use between EU countries and you are in no duty to prove that the items have been bought in the EU.
Noone will expect you to be awake on the bus to wait for any potential customs control. If the bus is stopped for a random check and one of the officers wants to talk to you, he will simply wake you up and do so.