At that time of the morning, the taxi ride from the Sheraton to BRU takes no longer than 20 minutes, and should cost around Eur25-30. There is a taxi rank outside the hotel and a doorman to help you hail one.

The train service to the airport takes about 15 minutes from Nord (Sheraton), 20 minutes from Central (le Meridien) and 25 minutes from Midi. Trains run every 15 minutes and fare is Eur2.90 one-way from any "Zone Brussels" station. A return is Eur5.80 and has a flexible return date : the ticket has a blank space where you write in your return date when you want to use it. I use this airport train service very often and have found it consistently reliable and uncrowded, even in rush-hour.