The body's biological clock is regulated by the doshas. In order to keep the tridoshas in a state of healthy equlibrium and digestion & metabolism (agni) in proper order, Ayurveda prescribes for each individual a specific daily routine ("dina"=day & "acharya" = behaviour).
The time of maximum activity of kapha is during early morning and every evening. The pitta period is during midday and midnight, while vata hours are dawn and dusk. Thus a pitta-type disease, like ulcers, may cause the most discomfort late at night.