Perhaps begin by telling them the parts of speech ...
sleep is a noun (something we need) or a verb (something we do)
asleep is an adjective which describes someone who is sleeping.
Then maybe give them a few example sentences:
I need some sleep (n). I �m so tired, I �ve got to sleep (v) soon. Don �t call me later, because I �ll be asleep (adj).
Then maybe give them a worksheet where they have to fill the blanks using �sleep � or �asleep � ...
eg
How long do you usually ________ for each night?
Were you _________ when the storm started?
I didn �t get much _________ last night.
Any help?
Brian