this is from the sids website:
Is it okay for my baby to sleep in a car seat?
Car seats are not ideal places for safe infant sleep in the home. Our advice is that the safest place for your baby to sleep ? both during the day for naps and during the night ? is in a crib, cot or Moses basket in a room with you for the first six months. It is important to check on your baby regularly when she is asleep. If your baby is being transported in a car, she should be carried in a properly designed and fitted car seat, facing backwards, and be observable by a competent adult. Be careful that your baby doesn?t get too hot and remove hats and outdoor coats when you get in the car. On long car journeys, stop for breaks for fresh air and a drink for the baby. Premature babies who may slump need particular care when travelling in a car seat.
A recent study of car seats gave this advice: ?We are still concerned about the vulnerability of infants in car seats ? We advise parents to use car seats that do not have a steeply angled back. Caregivers are also instructed that immature infants being discharged from the neonatal nursery should be watched carefully while in the car seat and transferred back to their cot as soon as possible after the car journey ends.? [Car seat test for preterm infants: comparison with polysomnography, Elder et al. Arch. Dis. Child. Fetal Neonatal Ed..2007; 92: F468-F472]