Stargate Atlantis (2004)

S4E4 Doppelganger

On a mission beyond their world, Colonel Sheppard unwittingly becomes the vessel for an alien organism. This entity begins to spread throughout Atlantis, impacting the crew by inducing horrifying nightmares. As fear grips the team, they must uncover the source of these disturbances and find a way to stop the alien influence before it leads to chaos within their ranks.