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If a routine EEG has been normal or has shown features which are suspicious but not definite enough to support a diagnosis of epilepsy, your doctor may ask for a sleep EEG. This is because abnormalities may become more apparent in the EEG during drowsiness and light sleep - particularly the abnormal brain activity seen in epilepsy.

A sleep EEG may be done in one of two ways. A sleep-deprived EEG, as the name suggests, is one carried out after a period of lack of sleep. Different EEG departments have their own particular routines and they will tell you exactly what they would like you to do in their appointment letter. Some ask people to try and remain awake the whole of the night before the test and will book an appointment for first thing in the morning. However, for many people this can be quite difficult. Some departments will ask you to stay awake for just part of the night - either by going to bed at the normal time and then setting the alarm to wake up at 3 or 4 am, or by staying up until about 4 am and then having just a few hours sleep before getting up again to go for the test.

The aim is to produce a state in which you are likely to fall asleep quite readily in the EEG department during the recording. The procedure for placing the electrodes on the scalp and so on is exactly the same as for a routine EEG. The initial part of the test while you are still awake will involve opening and closing the eyes, hyperventilation and photic stimulation. After this, the technician will ask you to lie down with your eyes closed and let yourself go to sleep. The room may be darkened by drawing the blinds and things are kept as quiet as possible. The recording is often rather longer - going on, perhaps, for an hour or an hour and a half - to give you a reasonable chance to drop off to sleep. Once the recording is over, the technician will gently wake you up.

Sometimes, despite depriving themselves of sleep the night before the test, people do not actually fall asleep during the EEG. This does not necessarily mean that it has been a waste of time, since sleep deprivation itself may bring out EEG abnormalities even if you remain awake.

A second approach is to allow a normal night’s sleep before the test and then give a drug to induce sleep before the EEG recording. This is a drug-induced sleep EEG. The drugs usually used are diazepam (Valium) and chloral hydrate. Again, each department has its own particular instructions which they will give you before you go for the EEG. You will often be asked to go without breakfast, as it is better for people to have an empty stomach if they are being given a sedative (calming) drug.

The drug will usually be given by mouth early on during the test and may take half an hour or so to act. Most people will fall asleep quite readily and will be able to wake up after one to two hours of recording. However, the way in which different people are affected by a dose of a sedative drug can differ quite widely. Occasionally, despite being given the standard dose, some people may remain awake during the test, while others may find it difficult to wake up after an hour or two and may need to rest in the department until they are alert enough to go home.

For both sleep-deprived and drug-induced sleep EEGs (and particularly the latter), you will normally be asked to bring someone with you who can help make sure you get home safely afterwards if you are still a bit sleepy.