All effects last only until end of turn. This includes the effects of Amulet of Sun.
The rules are unambiguous. From page 5:
1) Artifacts are Deeds.
If an effect tells you to gain a new Deed card (Advanced Action, Spell, or Artifact)...
2) Effects come from Deeds, and last only until end of turn.
Deed cards... provide a variety of effects that may be used on a given turn.
b. Other effects might allow you to modify the rules for the turn, or to gain something you wouldn't normally be able to gain...
If the effect modifies some values or rules, the change always lasts until the end of the current turn (unless stated otherwise).
3) Amulet of Sun is an Artifact:
Amulet of Sun
Gain a gold mana token.
If played during the Night, forests have their move cost reduced to 3, you can use gold mana, and you reveal garrisons of nearby fortified sites and all ruins as if it were day.
Same as the basic effect, except you get three gold mana tokens instead of one.