Why an active (working) DB session appears like SLEEPING process in UNIX?

Dears,

For more than 37 hrs, UNIX has been showing 15 processes in SLEEP state. The time in this state is increasing. These processes correspond to DB sessions.

The sessions in DB are changing their STATUS between ACTIVE and INACTIVE (v$session)  and v$session.LAST_CALL_ET doesn´t increase more than 30 seconds.

The wait events is changing between “SQL*Net message from client” and “db file scattered read”.

With this evidence I can say everything looks normal with the DB sessions, but I cannot explain to my user the reason about UNIX state of these processes is in SLEEP state.