For the most comfortable reach, your nightstand should sit level with the top...

https://www.red-bookmarks.win/the-golden-rule-for-bedside-tables-is-comfort-and-accessibility-aim-for-a

For the most comfortable reach, your nightstand should sit level with the top of your mattress or just 5-10 cm lower. This creates a natural motion when you grab your phone or glass of water at night