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Between 0300hrs and 0500hrs when the core body temperature is falling to its lowest, the urge to sleep is strongest. Also, during this time an individual tends to feel colder despite the surrounding temperature remaining the same.

Between 1300hrs and 1600hrs the core body temperature dips again. In Latin American countries this mid-day tiredness is recognised, and a 'siesta' - a short mid-day sleep - is an accepted part of their culture. In the UK, the traditional mid-afternoon cup of tea is the typically British way of coping with this dip in alertness.

 

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