This is actually a fascinating question, as schedules of eating and the choice of foods consumed relate directly to such things as blood sugar levels, which in turn relate directly to one's attenton span, acuity of thought, etc.
In my case, I had a woman PPL student of 51 years old who, in my non-medical, subjective judgement, was so weakened and inattentive due to strict adherence to a crash diet (she was trying to lose weight before her wedding), that I declined to let her solo. She eventually did finish her PPL.
Naturally, I am not drawing a comparison between such pathological self-starvation and the observance of Ramadan.
atchoum's question is, in itself, quite interesting, and I hope folks out there can shed some light on it.