Sentence stress is a characteristic of connected speech. i.e. spoken language all the words join to make stream of sounds. Some other characteristic of connected speech are contractions and vowel shortening in unstressed, word and syllables, e.g. the schwa sound. These characteristic help to keep the rhythm of speech regular. The regular beat falls on the main stress, while the weaker syllables and words are made shorter to keep to the rhythm. Try saying the sentence above and beating out a regular rhythm on your hand as you say them.