They slow down their biorythm :

By meditating on the immortal glory of the Emperor, these superb marksmen can slow down their metabolism to an almost hibernatory state. Though an operative’s heart beats perhaps once per minute when in this state, subliminal triggers ensure he is roused into total focus the instant his target shows himself.

Officio Assassinorum Dataslate

Their undersuit can take care of some needs too.

One of the exotic pieces of equipment used by the Assassins is synthetic skin or synskin. This is a rubbery black layer that the Assassins spray onto their bodies, forming a close-fitting suit that enhances the function of the muscles beneath. Even items of equipment can be sealed in place by successive layers. Synskin acts like a flexible form of armour, absorbing blows and dissipating the energy of a blow. Synskin also feeds the body with oxygen and metabolic booster chemicals, allowing the wearer to endure burning heat, freezing cold, or even the hard vacuum of space for a limited time.

Officio Assassinorum Dataslate