at once vs early

at once

adv
  • Immediately; now; right away. 

  • At the same time; simultaneously. 

early

adv
  • At a time before expected; sooner than usual. 

  • Soon; in good time; seasonably. 

noun
  • A shift (scheduled work period) that takes place early in the day. 

adj
  • Near the start or beginning. 

  • Near the start of the day. 

  • Having begun to occur; in its early stages. 

  • At a time in advance of the usual or expected event. 

  • Arriving a time before expected; sooner than on time. 

  • Of a star or class of stars, hotter than the sun. 

How often have the words at once and early occurred in a corpus of books? (source: Google Ngram Viewer )