The 19-year-olds joined the Spireites on an initial one-month's loan and immediately helped the League Two side to three successive victories in the race for promotion.
Striker Dowson scored within four minutes of his Chesterfield debut against Brentford, while Hartley has turned in combative performances at the back, though it will be of some concern that he has already accrued three bookings.
Spireites manager Lee Richardson said: "We are pleased that the two Sunderland lads are staying with us some more as they continue to provide healthy competition in the squad. This helps keep everyone on their toes."
The duo both signed professional contracts with the Black Cats in 2007.