ORA-4031 errors in the night

Since a few days I get ORA 4031 messages in the night, when the database is not doing mutch.

I've tried to flush the shared pool, that didn't help.

Anyone? See below part of the trace file.

Incident 541200 created, dump file: /oracle/diag/rdbms/kewillp/kewillp/incident/incdir_541200/kewillp_m000_18340_i541200.trc

ORA-04031: unable to allocate 4160 bytes of shared memory ("shared pool","unknown object","sga heap(1,0)","modification ")

*** KEWROCISTMTEXEC - encountered error: (ORA-04031: unable to allocate 4160 bytes of shared memory ("shared pool","unknown object","sga heap(1,0)","modification ")

)

  *** SQLSTR: total-len=190, dump-len=190,

      STR={insert into wrh$_sys_time_model   (snap_id, dbid, instance_number, stat_id, value)  select    :snap_id, :dbid, :instance_number, stat_id, value  from    v$sys_time_model  order by    stat_id}