  We study the Lamb shift of both freely-falling and static two-level atoms in
interaction with quantized conformally coupled massless scalar fields in the de
Sitter-invariant vacuum. We find that the Lamb shifts of both freely-falling
and static atoms are in structural similarity to that of an inertial atom
immersed in a thermal bath in a Minkowski spacetime. For the freely-falling
atom, the Lamb shift gets a correction as if it was immersed in a thermal bath
at the Gibbons-Hawking temperature, thus revealing clearly the intrinsic
thermal nature of de Sitter spacetime. For the static atom, the Lamb shift is
affected by a combination of the effect of the intrinsic thermal nature of de
Sitter spacetime and the Unruh effect associated with the inherent acceleration
of the atom.
