The top bunk bed has a saftey rail, naturally that prevents the user from falling off. Most bunk beds are designed for thinner 15cm mattresses. We have designed ours to accept a 20cm thick mattress.

With a 20cm this enables the guard rails to be effective in line with bunk bed regulations, if you use a 25cm mattress as an examply the rails are less effective. So, you can use a thicker mattress but this would not be recommended and subject to your own risk. The other factor is the user, if they are responsible then this would lessen the risk.

Remember, this only applys to the top bunk, the bottom bunk can be any height you want given there are no safety rails.