How come anti-psychotics make you feel tired?

Dopamine blockade: There are actually probably several ways they can produce sedation including blockade of Dopamine receptors or blockade of serotonin receptors. As a corollary to this you can also see how stimulants that increase Dopamine would make you feel the opposite of tired. This is likely oversimplified although from a conceptual point of view this may provide a framework for understanding this phenomenon.

Many are sedating: In addition to the Dopamine issues my colleague described, many antipsychotic medicines also have sedating qualities that make you feel sleepy. People are variably sensitive to this, and may feel tired as a result.