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

https://fionafreshmaids.com/the-ultimate-guide-choosing-the-perfect-bedside-table-height-for-a-65-cm-mattress/

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