Don't know about 'should' as each person is individual. In general, I find my sleep time / length varies naturally through the year / seasons. I sleep longer in the winter (dark evenings) and far less in the summer (much lighter). That is how we have evolved, surely?
The pineal gland (one of the endocrine glands) is involved in modulation of wake / sleep patterns and seasonal functions, as are various other parts of the central nervous system, and the eyes. Indeed, the production of melatonin by the pineal gland is stimulated by darkness and inhibited by light.
Maybe the answer is: in an ideal world one's body can decide for itself, given the changes in seasons (natural light). Is this what we ought to aspire to? Learning to really listen to our body as it naturally follows the rhythms of nature?