Announcing your excuse before the test is a bad trade. Do well and it’s wasted. Do badly and nobody buys it.

Pre-announcing why it might go badly is a trade that loses in both directions. Do well and you have given away your own result, since you told everyone it should not have happened. Do badly and nobody believes the excuse anyway, because you had it ready before the event, which is the one thing that makes an excuse sound untrue.

Keep the explanation to yourself until there is a result to explain. If you are genuinely unprepared, use the time you were going to spend warning people. Nobody needs to know how much sleep you got.