How? There is a whole industry built on poor sleep, but the key to understanding it lies in the circadian rhythms which govern all of life.
As a junior doctor, I trudged through post-take ward rounds, longing to climb into bed alongside any of the patients, even the most scabrous and sick, just for a moment’s rest.
After a week off I would cope with the first night on-call, feeling “this isn’t too bad”. The next night my grip was weakening, by the third I was beyond remembering what normal life was like.