How can somebody be resurrected if they are cremated?
The Bible does indeed use the analogy of sleep to describe the state of the dead (1 Thessalonians:4:13; 1 Corinthians:15:51, etc.). Those who have died are totally unaware of the passage of time and feel no pain or sorrow (Ecclesiastes:9:5, 10). They will be "awakened" at the time of the resurrection. But this does not require that their physical bodies persist until that time.
Scripture reveals what happens to our physical bodies after death. Genesis:3:19 tells us that we will return to dust. If moisture and air are present, even our bones will eventually disintegrate. In the final analysis, there is no difference between the dust of decomposition and the ashes of cremation.
Neither will have any effect whatever on God's sure promise to resurrect all human beings in due course according to His plan and purpose (1 Corinthians:15:22; John:5:28-29; Revelation:20:4-6, 11-14)—which must include those who have been cremated.
Faithful Christians will be resurrected with spirit bodies (compare 1 Corinthians:15:35, 45, 49). The rest of mankind who died in past ages, beginning with the nation of Israel, will be reconstituted in the flesh (see Ezekiel 37) before being offered immortality.
On the overall questions of death and the future, you'll find more details spelled out in our free booklets What Happens After Death? and Heaven and Hell: What Does the Bible Really Teach?