"Sleep" is occasionally used to refer to the death of believers (1 Thessalonians 4:13). At this point, Lazarus had already been dead for three days (compare John 11:39), since it would take about a day to walk from "beyond Jordan" (John 10:40), where Jesus was staying at the time, to Bethany. Jesus knew that He could not have reached Lazarus before he died, even if He had started as soon as the sisters' messenger reached him (John 11:3). His purpose in waiting was to demonstrate beyond question both His omniscience and omnipotence (John 11:4,15).

Source:

KJV Defenders Study Bible, by Dr. Henry Morris, Ph.D.
Publisher: Thomas Nelson
Language: English
DEF 10 ISBN 0-529-10444-x
DEF 10-1 ISBN 0-529-10445-8