Happy or Stress

Should design and creative work of art be done in a stressful condition, or a happy mood?

None of them.

Create in neutral state of mind - not too happy, not overwhelmingly excited and certainly not in stressful condition.

Emotion like anger, happiness can create drive; similarly, stress can refrain, force or effect thinking and decision-making.

Whatever requires drive is not sustainable.

Drive dependent also means no creative work when there is no drive. 

So eventually, to create, you have set mood, wait something strikes you - deadline and stress. Drug is much worse. Some of the thought might not even true, bias as they are all factor-driven. Fatal drawback may occur after factor is gone. 

None of these are sustainable. Control design based on research, truth, test and back-and-fort thinking instead of driving it with excitement and anxiety. 

Develop mentality: Open-mindedness, mode of thinking, mindfulness, instead of depending on mood.

Discover mental toolsMental accounting, questioning, honest thinking, instead of depending on external factor.

These are much applicable in long run than short-term gratification in getting ideas or things done.