In Europe during the 1900's three independant scientists - working with people and observing their health, moods and intelligence - found that there were exact cycles of events which matched these three aspects of our being, and that these three cycles repeated themselves with astonishing accuracy. These were called Biorhythms. The three rhythms are:

PHYSICAL RHYTHM - 23 day cycle. Centred around muscular functions. 
SENSITIVITY / MOOD RHYTHM - 28 day cycle. Centred on nervous functions.
INTELLECTUAL RHYTHM - 33 day cycle. Centred on cerebral functions.

Biorhythms concern the physical, sensitivity and intellectual aspects of our being, and the energies of these aspects vary from day to day. These rhythms are best displayed as an undulating curve. Used wisely, a biorhythm chart can be extremely useful in understanding (and coping with) what are often referred to as 'off days', as well as providing explanations for several
well-used idioms...

'I'm all fingers and thumbs today...'

'Got out of bed on the wrong side this morning...'

'I can't think straight at the moment...'