Usually, speaking about metastable states in atoms we assume that those decay very fast, say, faster than a microsecond. But not in a Helium atom - its metastable state lives for extremely long time.
In the ground state helium has two electrons on the 1s shell, one of which can be excited to the 2s shell, using inelastic electron scattering. In such a way the metastable helium He* is obtained. The thing is that the transition 2s->1s is dipole forbidden, and the lifetime becomes huge: a recent investigation of Hodgman with colleagues shows it to be 7870 seconds, which is longer than 2 hours.
Well, that's a good opportunity to take some sleep during the experiment.