at.algebraic topology - Covering maps on Euclidean spaces and spheres

I think Pete should have made his comment an answer, so I'll do it for him.

Theorem 1.38 of Hatcher's Topology says that connected coverings of a (locally path-connected, and semilocally simply-connected) topological space $X$ are in bijection with conjugacy classes of subgroups of $pi_1(X)$.

Since $pi_1(X)$ is trivial for $X=mathbb{R}$ or $X=S^n$ ($n>1$), there are no connected coverings.