The Tributes will not be warned for when their injections will occur. For those who are known to be compliant and willing to work with the capitol, they may be taken aside and told they are getting a shot or whatever else may convince them. Others may be injected within their sleep (and have been drugged earlier to keep them that way).
If you are not a tribute, your injection comes on your decision, having cashed in your ticket at Hypnogogia for a good rest and stay.
Everyone is ushered to bed at once and everyone will be quick to discover why.
The first effect is a sort of paralysis - not the terrifying inability to move, but a signal to the brain that says
why move? Moving is so much effort. It's quickly followed by drowsiness, and then a chill that radiates from the needle into the body, and finally, unconsciousness.
This shared dreaming carries on whenever you sleep for seven total days, with the Expos running during their waking hours. Those with Vistors will meet them within the dream.
( Some of the want to use you, some of them want to get used by you... )