[PetitSasquatch]

Nothing that I know of has the same effect as medically prescribed stimulants.

However, I've found consistently that high quality saffron (Affron TM) extract has a mild SSRI quality, which when used during the seratonergic phase of your day (i.e. after lunch) can be noticeably effective in normalizing mood, limiting depression and generally getting into "the zone" or flow with certain types of work, like writing etc..

There are some papers on this, mostly from Iran I think.

As for non-medicinal, as cold shower or bath before starting work can really help breaking through the wall.