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 tools: Mental 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.