Claims 1. A pharmaceutical composition comprising zolpidem or a salt thereof characterised in that it consists of a controlled-release dosage form adapted to release zolpidem or its salt over a predetermined time period, according to a biphasic in vitro profile of dissolution when measured in a type II dissolution apparatus according to U. S.

Pharmacopoeia in aqueous buffer at 37°C, where the first phase is an immediate release phase and the second phase is a prolonged release phase.

2. A pharmaceutical composition according to claim 1, characterized in that the first phase has a maximum duration of 30 minutes.

3. A pharmaceutical composition according to claim 1 or 2 characterized in that the second phase has a profile of zero order release.

4. A pharmaceutical composition according to claim 1 or 2 characterized in that the second phase has a profile where the release is proportional to the square root of the time.

5. A pharmaceutical composition according to claim 1 or 2 characterized in that the second phase has a profile of first order release.

6. A pharmaceutical composition according to claim 1 or 2 characterized in that the second phase has a sigmoidal release profile.

7. A pharmaceutical composition according to any one of claims 1 to 6 characterized in that 40 to 70% of the total amount of zolpidem is released during the immediate release phase.

8. A pharmaceutical composition according to any one of claims 1 to 7 characterized in that the time for release of 90% of the total amount of zolpidem is between 2 and 6 hours.

9. A pharmaceutical composition comprising zolpidem or a salt thereof characterized in that it comprises two kinds of pharmaceutical entities: one immediate release entity and one prolonged release entity.

10. A pharmaceutical composition according to claim 9 characterized in that it consists in a dosage form chosen among capsules, tablets, multilayer tablets, multicoated tablets.

11. A pharmaceutical composition according to claim 9 or 10 characterized in that it consists of a capsule comprising one or more immediate release tablets and one or more prolonged release tablets.

12. A pharmaceutical composition according to claim 9 or 10 characterized in that it consists of a capsule comprising a mixture of prolonged release pellets and immediate release pellets.

13. A pharmaceutical composition according to claim 9 or 10 characterized in that it consists of a tablet comprising a number of prolonged release coated pellets comprising the drug imbedded in a matrix also comprising the drug.

14. A pharmaceutical composition according to claim 9 or 10 characterized in that it consists of a multilayer tablet comprising: (i) one or two prolonged release layers, comprising the drug and a hydrophilic polymer (preferably a cellulose derivative), (ii) one or more immediate release layers comprising the drug, and possibly, (iii) another layer not comprising the drug, but comprising hydrophilic polymers.

15. A pharmaceutical composition according to claim 9 or 10 characterized in that it consists of a multicoated tablet comprising: (i) a core comprising the drug and optionnally a pharmaceutically acceptable organic acid to maintain constant pH, (ii) a polymer coating layer giving slow release of the drug from this core, (iii) a coating layer comprising the drug which is released rapidly or immediately on contact of the dosage form with fluid.

16. A pharmaceutical composition according to claim 9 characterized in that the immediate release entity and the prolonged release entity are administered simultaneously but separately.

17. A pharmaceutical composition according to any one of claims 1 to 16 characterized in that the prolonged release entity comprises a pharmaceutically acceptable organic acid which can be chosen among tartaric, malic, fumaric, lactic, citric, adipic or succinic acid and their acid salts, in the form of racemates or isomers.

18. A pharmaceutical composition according to any one of claims 1 to 17 characterized in that it is capable of generating visual change if it is introduced into a drink, whether or not containing alcohol, 19. A pharmaceutical composition according to claim 18 characterized in that the visual change consists of release of included colouring excipients, or floating of the composition at the surface of the drink, or formation of insoluble particles on the surface of the drink, on the brim of the glass, in the drink and/or on the bottom of the glass or a combination thereof.

20. A pharmaceutical composition according to claim 19 characterized in that it comprises an effervescence generator, a hydrophilic excipient and optionnally a lipophilic excipient and a colouring excipient, taking the form alternatively of a tablet, a multilayer tablet or a capsule containing controlled release pellets and immediate release pellets or granulates.

21. A pharmaceutical composition according to any one of claims 1 to 20 characterized in that zolpidem has the form of zolpidem hemitartrate.