Morning, afternoon, evening and night are defined as different blocks of time within a 24 hour day. It is generally accepted that morning ends at 12 p.m. Afternoon, evening and night have more subjective boundaries.
Technically, morning can be defined as any time that falls between 12 .a.m. and 11:59 a.m. Some definitions, such as the one on Dictionary.com, also accept morning as beginning at dawn.
Afternoon begins at 12 p.m. and continues for several hours. The point at which afternoon ends and evening begins is more difficult to define. Some definitions describe evening as the period of time between the evening meal and bed time. It may also be described as a period of time beginning when sunlight begins to fade and ending when the sun has set completely and total darkness sets in. Evening may begin at different times depending on the season. For example, because the sun sets earlier in winter, evenings in that season may begin around 5 p.m. as opposed to summer evenings, which may begin at 6 or 7 p.m.
Dictionary.com describes night as the period of darkness between sunset and sunrise. Night may be longer or shorter depending on the season, as the sun sets earlier in winter than it does in summer.