A bedside table should sit 5 to 10 cm below your mattress-top for the most...

https://amumreviews.co.uk/should-bedside-tables-be-higher-than-the-bed/

A bedside table should sit 5 to 10 cm below your mattress-top for the most comfortable reach. Fumbling upwards or reaching down feels awkward when you are tired. Most UK beds sit quite high, so aim for a table between 55 and 70 cm tall