This poem "All You Who Sleep Tonight" is written in iambic trimeter, meaning that every second syllable is stressed (as I've marked in bold below). There are three iambs per line (each with one unstressed and one stressed syllable); that's why it's called trimeter.
Remember that, when reading the poem aloud, the accented syllable is not always strongly emphasized, and sometimes the meter, for the sake of slight variation, can be a bit off (as in the second line below, where "far" really gets its own emphasis, as does the word "some" in the seventh line) :
All you who sleep tonight
Far from the ones you love,
No hand to left or right
And emptiness above -
Know that you aren't alone
The whole world shares your tears,
Some for two nights or one,
And some for all their years.
The meter in this case adds to the meaning because its steady rhythm creates a soothing effect, which matches the words of the speaker as he tries to offer encouragement and solace to those who are feeling along and missing the person they love.