Does intention (niyyah) have to be consciously formulated before every act of worship?

In Islamic practice, niyyah (intention) is a fundamental part of many acts of worship, such as prayer, fasting, Hajj, and zakat. However, there seems to be some variation in how intention is understood: Some scholars emphasize conscious and explicit intention before every act, while others say that the mere decision to perform the act at the proper time is sufficient, since Allah knows what is in the heart. Is it necessary to consciously formulate the niyyah before each act of worship, or is an internal, implicit awareness enough? For example: If a person wakes up for Fajr, performs wudu, and begins praying without explicitly stating the niyyah is the prayer valid? In fasting during Ramadan, is waking up for suhoor and abstaining from food enough to count as a valid fast, even if one forgets to explicitly think, “I am now fasting”?