Peace be upon you. The intention in acts of worship, in general, is the intention to perform the act as an act of devotion to Allah. In other words, you should know that you are praying the night prayer and doing it as an act of devotion to Allah, such that if you were asked what you are doing, you would answer, 'I am praying the night prayer.' It is not necessary to say anything either in your heart or on your tongue. Therefore, simply standing up to perform the night prayer and your determination to do it with the intention of drawing closer to Allah is what is meant by intention.