The investigators wish to investigate whether enhancement of SWS, which is seen after a drug called sodium oxybate, reduces the impact of sleep disruption in CFS on daytime function, specifically sleepiness and mental performance. This is a safe and well-tolerated drug that is licensed for excessive daytime sleepiness (EDS) and cataplexy associated with narcolepsy.

The investigators will study 12 patients diagnosed with CFS using international diagnostic guidelines. The investigators will record overnight sleep with EEG (brainwave) measurement on the 1st and 4th nights of a 4 night period during which sodium oxybate and placebo will be taken nightly, and the investigators will measure next-day sleepiness, mental performance and fatigue, and compare drug and placebo nights.

Chronic fatigue syndrome (CFS), characterised by chronic disabling fatigue, sleep impairment and other symptoms, is associated with neither a currently identifiable disease process nor major psychiatric illness, and has an estimated prevalence in primary care of 1-2%. Sleep impairment is common in nearly everyone with CFS, with both daytime sleepiness and unrefreshing nighttime sleep reported, and consequent impact on daytime function. It may be that fundamental regulatory processes that control sleep are disturbed in CFS, leading to different effects on sleep and daytime symptoms depending on the subject's prior sleep, daytime routine, medication and other factors. The investigators contacts with patient groups have indicated that patients are generally confident that on days when their sleep is better they perform better in the day. There is growing evidence that deep, slow wave sleep (SWS) is altered in CFS, and this may suggest impairment of build up of sleep pressure during the day.

Inclusion criteria

• Meeting criteria for CFS according to both the revised CDC (Fukuda 5) and Canadian diagnostic systems.
• Aged 25-65.
• Good grasp of the English language.

Exclusion criteria

• Taking any of the following medication: opioids, tramadol, phenytoin, valproate, ethosuximide, benzodiazepines, zolpidem, zopiclone, zaleplon, antidepressant except <30mg amitriptyline, or any other medications likely to interact with sodium oxybate or with sleep in the opinion of the investigators.
• Current major psychiatric disorder.
• Unusual sleep schedule; (bedtime routines that fall outside 9 p.m. to 10 a.m.; usual time in bed > 12 hours).
• Pregnancy, lactation or being female and not using reliable contraception.
• Relevant abnormal clinical findings at screening visit.
• Taken alcohol in the 24 hours before each study visit or drugs of abuse in the week before each study visit

2 months