Photo taken on board the International Space Station. With no force acting on the body, the astronauts can align in any direction.
Picture your body on a 3D Cartesian coordinate system with the Y-axis running parallel lengthwise along your back, the X-axis parallel to your shoulders, and the Z-axis going into the plane formed by the surface of your stomach. Your spine has 33 vertebrae that are separated by cartilage. During the day, gravity acts along the Y-axis, causing compression in the spine as well as joints such as knees and ankles. This compression occurs whether you are sitting or on your feet.
The total compression of all cartilage points over the course of the day has an average displacement of -1 cm in the Y direction. When you sleep gravity acts either in the Z direction if you sleep on your back or stomach, or the X direction if you sleep on your side. The absence of forces acting in the Y direction allow your spine and joints to decompress, gaining back the half inch or so that was lost during the day. Because of this, you are taller in the morning that you are at night.
Astronauts in space are in a microgravity environment where they experience virtually no force. This means that their bodies actually expand on a macroscopic scale. For example, astronauts can gain up to 3% in height while in space.
Humans are built to thrive in earth’s environment and have systems in place that make sure fluids are distributed evenly around the body instead of pooling in the lower half as a result of gravity’s influence. In space these systems still function, causing more fluids to congregate in the upper half of the body. This is responsible for the puffy-faced appearance many astronauts get.
Over time in a micro-g environment, muscles atrophy and cardiovascular strength decreases because the body is not experiencing resistance. After long periods of time bone loss is also experienced. To manage the amount of change their bodies experience, astronauts have daily exercise regimes. The International Space Station has equipment that allows astronauts to perform exercises similar to those they could do in a gym on earth.
The lack of gravity also means that routine tasks like showering have a different process. In microgravity liquids like water exhibit gas-like behavior, propagating in small, particle-shaped packets. Showering in space involves containing the weightless water particles and vacuuming the excess from the air to prevent water from spreading around the area.