Almost one in four young men skipped doctor and hospital appointments on Mondays, a study has found.
Psychologists at Glasgow University found the NHS could save tens of millions of pounds a year if it targeted medical appointments to patients' profiles.
It suggested hospital outpatient and GP appointments could be arranged later in the week for young people and earlier in the week for older patients.
The study revealed that around 12% of all out-patient appointments at UK hospitals are missed, costing the health service an estimated £600 million a year.
The trend, for attendance to dip at the start of the week and steadily rise until Friday, was blamed by the psychologists on people's "emotional associations" with certain weekdays.
Dr Rob Jenkins, senior lecturer in psychology at the university, said: "Mondays have the most negative response, Fridays the most positive. And emotional tone brightens steadily over the intervening days.
"Missed appointments seem to follow the psychological peaks and troughs of the weekly cycle, with emotionally positive days boosting patient resilience and improving attendance.
"This interpretation chimes with many of the psychological reasons patients give for missing their appointments – fear of bad news, fear of unpleasant treatment."