A SAILPLANE PLT SUFFERED HYPOXIA WHICH CONTRIBUTED TO HIS LNDG AFTER DARK.

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I WAS DOING HIGH ALT GLIDER FLYING IN A MOUNTAIN WAVE. THROUGH PRE-AGREEMENT WITH LEESBURG; VA; FSS; THE PRE- DESIGNATED WINDOW WAS OPEN TO 23000 FT. DURING THE FLT; THE HOSE SUPPLYING OXYGEN BECAME DISCONNECTED FROM THE MASK. AS IT TOOK ME A WHILE TO REALIZE THIS; I DID NOT PAY CLOSE ENOUGH ATTN TO TIME AND STARTED MY DSCNT LATER THAN I SHOULD HAVE; AND LANDED APPROX 20 MINS AFTER SUNSET AT GRANT COUNTY ARPT. I FEEL THAT THIS WAS CAUSED BY 2 THINGS. ONE BEING THE TUBE SLIPPING OFF THE MASK AND TWO: MY NOT VIEWING THAT PORTION OF THE MASK AS A DISTINCT CONNECTION. THE MASK -- ONE NO. X HIGH CONCENTRATION SEE-THROUGH ADULT OXYGEN MASK; PATENT NO. Y. THE CLR PLASTIC OXYGEN DELIVERY TUBE IS ATTACHED TO THE MASK BY SLIDING OVER A PLASTIC NIPPLE. AT FIRST GLANCE; IT APPEARS TO HAVE BEEN GLUED OR SEALED ON. IF THE TUBE IS TUGGED ON WITHOUT TWISTING; THERE IS A BIT OF RESISTANCE. HOWEVER; IF THE TUBE IS SLIGHTLY TWISTED ON THE NIPPLE; THE TUBE EASILY SLIDES RIGHT OFF. ALTHOUGH THE DIRECTIONS IMPLY THAT THE MASK INS INTENDED FOR MEDICAL USE; IT IS DISTRIBUTED IN A CLR PLASTIC BAG MARKED AVIATION OXYGEN ON A LABEL. SINCE THIS MASK USES A CONNECTION THAT APPEARS SEALED BUT EASILY CAN SLIP OFF; THIS MASK SHOULD BE PROHIBITED FROM BEING USED IN AVIATION FLT BY PLTS. CONNECTION SHOULD BE MORE ROBUST. THIS MASK MAY BE OK IF A LIP ON THE NIPPLE AND CLAMP OVER THE HOSE WERE USED. THIS HOSE CONNECTS TO THE NIPPLE AT A LOCATION ON THE MASK BELOW THE WEARER'S MOUTH WHICH IS ALSO OUT OF SIGHT FROM THE PERSON'S VIEW. THE PLT -- FOR OXYGEN SYS; WE ARE TAUGHT PRICE -- TO CHK PRESSURE; REGULATOR; INDICATOR; CONNECTIONS; EMER BAIL OUT BOTTLE. AS THE PLT; I FAILED TO CHK THE TUBE WHERE IT JOINED ONTO THE NIPPLE OF THE MASK AS BEING A CONNECTION. I ERRONEOUSLY SAW IT AS A SEALED CONNECTION AND DID NOT CHK IT AS FREQUENTLY AS I SHOULD HAVE.