Beings from a pocket of subspace who secretly abducted Enterprise-D crew members for medical experiments while they slept.
The Solanae are beings who live in a region of subspace outside normal space. They came into contact with the Enterprise-D when the ship's experiments created an opening into their realm, and they began taking crew members from their beds to study them.
In the episode Schisms, William T. Riker and others suffered exhaustion, strange sensations and missing time. Deanna Troi led the victims in recreating their shared memories on the holodeck, which revealed an alien examination room. Geordi La Forge and Data found the source in the cargo bay, Riker escaped during a later abduction, and the crew sealed the rift to stop the Solanae from returning.