It is well known that a mass and acceleration can have the same "gravity effect" on an object. Let's assume that an astronaut is being accelerated in a spaceship for a long time by 9.8m/s, which I presume means that he will feel the same gravity as near earth. But when approaching the speed of light, the effect of energy turning into mass is strongly visible. Which mean that the change in speed will decrease? Although the same amount of energy is accelerating the spaceship. Will the astronaut feel less "gravity" when approaching the speed of light?