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Last words past and present at newscientist.com/lastwordTHE LAST WORD

Chaotic kipLast night I slept really well but today I have spent the whole day feeling tired. On the other hand, I sometimes have a poor night’s sleep and I am just fine the next day. How can this be?

n Your brain may have slept well last night, but did your body get adequate rest, especially if you went to bed tense with worry? In her book Stress and Relaxation, Jane Madders talks of people who go to sleep so keyed up, with their jaws and fists clenched tightly, that it’s no wonder they wake up exhausted.

Studies looking at the electrical conduction system of the heart during physical workouts have shown that muscles have plenty of reserve for continued contraction even after a person feels they have reached their limit. And it’s common for athletes to experience a “deep tiredness” after very intensive training – even at times in the circadian rhythm when the brain is set for wakefulness.

It transpires that feelings of tiredness are a brain construct, which is separate from the hard-wired daily sleep cycle. Indeed we might make some sense of the chaos we sometimes feel by thinking of sleep as a means of refreshing the brain itself, and tiredness as a separate function that protects the body and organs from genuine overwork by forcing us to rest.

A related factor behind feeling drained the next day is low

overnight blood sugar. This is more likely after a physically active day because the muscles will take more sugar from the blood to replenish their glycogen store. Alcohol consumption earlier in the evening can cause overnight lows through its delayed effect on glucose release by the liver. To combat this have a light carbohydrate snack or non-

stimulant milk drink before bed to trigger an insulin surge, but make sure you avoid sugar.

On the other hand, did you really sleep poorly when you felt fine the next day? In one study,

insomniacs who had complained of lying awake all night were actually observed to have had several spells of sleep.

Nothing has caused more sleep loss than fretting over whether you are getting enough, but it has been shown that bright awakenings depend more on the phase in the sleep cycle you wake from than on the total amount of sleep you get.

You will emerge fresh from the lightest phase of sleep, called type 1, and from the light dreaming sleep that intersperses successive sleep cycles. A night dotted with catnaps each shorter than 15 minutes (ending before you enter deeper sleep), or a single stretch of one-and-three-quarter hours (thus ensuring you have returned to type 1, having gone

through the deeper and restorative types 2, 3 and 4), may provide all the sleep and bodily rest you need. But an alarm clock that wakes you during type 4 sleep could see you groggy and grouchy all morning.

Waking with the feeling you need more and more sleep – known as “sleep fat” – can sometimes result from too much sleep. In these ways, sleep and rest, and shortfalls thereof, can act either for or against us.Len WinokurLeeds, UK

This week’s questionsDream onWhy do I have recurring dreams, years after I left university, of being about to sit an exam but knowing nothing of the subject matter? I’m not alone, lots of people I speak to have the same. Guy JonesAltrincham, Cheshire, UK

Fingers oF soLCan anyone explain the pattern seen here in the sky (left)? It was spotted in the Pyrenees at about 8 am. We were heading east and although it gradually faded, it lasted for some time. The mountain pass over which the light is shining is at an altitude of 2000 metres and there is nothing in the next valley or mountain that could explain the pattern. Simon HoughtonSan Sebastian, Spain

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“insomniacs who complained of lying awake were actually observed to have spells of sleep”

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