Motivation and Anxiety
According to Hebb's classic formulation (1955), optimal "hedonic tone" or sense of well-being, and hence performance in learning, is achieved at a moderate level of arousal. Too little arousal leads to boredom, and too much to anxiety, both of which inhibit effective performance.
This model serves quite well for present purposes, but has been effectively criticised and modified by Apter (1989). He distinguishes two modes of experience—one is oriented towards excitement (with boredom as its opposite pole), and the other towards relaxation, with anxiety as its opposite pole.
The so-called "accelerated learning" movement, on the other hand, argues that learning is best achieved in a state of minimal anxiety and relaxation. However, in the words of one of their sites: "Accelerated Learning is a proven method of increasing the absorption of knowledge." That is a rather limited conception of learning, so even if Lozanov's (its founder's) theories are correct their applicability may be limited.
This topic exposes some of the major limitations of using the web as a research library. You will get many “false positives” (people hyping their educational snake oil), and quite a few “false negatives” (people illegitimately rubbishing stuff on the basis of their own prejudices). You may get some true positives (which people want to tell you about), but true negatives are rare, because the language is that of "failure" to demonstrate an effect.
Don't trust. Triangulate. And that goes for this site as well!
A fuller discussion of motivation should also cover Deep and Surface learning.