杏吧原创

The big sleep

Question: Why do you feel tired in the morning after a night鈥檚 sleep of
perhaps 10 hours, but after a few hours鈥� sleep you feel wide awake?

Answer: The answer to the question depends first on how much sleep you needed
that night. If you are catching up on a lot of lost sleep鈥攁nd we usually
need to recoup one-third of the sleep we have lost鈥攖hen a good 10 hours鈥�
sleep would result in your feeling refreshed and alert. If, however, you do not
need the full 10 hours鈥� sleep but still take it, then 鈥渨orn-out syndrome鈥� may
result. This is a type of sleepiness more akin to fatigue and can take several
hours to recover from. However, you can gradually increase the amount of sleep
that you take, so that 10 hours becomes usual, and not experience this
fatigue.

In one experiment by Timothy Roehrs, reported in the journal Sleep in
1989, people who were generally sleepy and another group who were generally
alert increased their sleep length to 10 hours per night for six nights. Only
the originally sleepy subjects had an increase in alertness.

Taking only a few hours鈥� sleep on one night may not result in poor
performance or a bad mood, but if this sleep deficit is repeated on successive
nights the cumulative sleep debt will cause sleepiness. Because deep sleep is
obtained mainly in the first few hours of the night, and seems to be more
important than dreaming or rapid eye movement sleep, which occurs later in the
night, sleep of a few hours can be enough to feel fine the next day.

Sometimes, depriving someone of REM sleep alone, by only allowing them to
sleep for a few hours, can make them more alert, and has been used in the
treatment of depression.

Mark Blagrove

Department of Psychology

University of Wales, Swansea

Answer: Waking up feeling tired after a long sleep is quite common and may be
caused by sleep apnoea. In some people the upper respiratory tract closes as
they relax into deep sleep. The brain registers this and realises the person
must wake up to start breathing again. So the brain brings the individual out of
deep sleep into lighter sleep of poorer quality. This happens repeatedly over
the course of the night and consequently sleep is very fragmented and the usual
refreshing 90-minute cycle from light, to deep, to REM sleep, cannot be
established. Similar problems can occur in individuals who have taken alcohol or
drugs.

However, an individual can sleep for perhaps only 180 minutes, completing
only two full sleep cycles without interruption, and feel fine on waking. This
shows that quality of sleep is more important than quantity.

Tom Mackay

Sleep Laboratory

Edinburgh Royal Infirmary

Rope trick

Question: Under what circumstances are ropes and braids actually stronger
than the same number of individual fibres and why?

There are several ways in which ropes of twisted fibres behave differently
from a collection of individual fibres. Two different aspects are explained
below: how friction between fibres adds strength, and the way in which twist
spreads load among a number of fibres鈥擡d

Answer: Ropes, braids and woven fabrics can sometimes exhibit strength
greater than the individual strengths of the same number of fibres combined in
any particular direction. When fibres are tested for tensile strength, weak
spots along the length of the fibre will tend to form weak links鈥攖hese are
the areas where the fibre will break.

But in a rope, the weak spots in the fibres become randomised along the
length of the rope. The twist in the rope increases fibre-to-fibre pressure
perpendicular to the axial rope direction. The resulting increase in friction
enables adjacent fibres without weak spots in the same area to add strength to
each other. Normally, optimum strength in a rope is achieved when the twists in
the fibres鈥� single-thread, ply and cabled construction are axially oriented in
the direction of the rope axis. Too little twist will not develop adequate
compressive force and resulting friction to achieve optimum strength. Too much
twist will place the fibres鈥� single-threads, ply or cable in a state of shear
and this too will reduce the rope鈥檚 strength.

In the case of some braids and woven fabrics, the interlacing of yarns causes
compression where one perpendicular or angular yarn crosses a yarn in the
opposite direction. With braids and woven fabrics for a given size and type of
yarn, there is an optimum construction that will maximise yarn-to-yarn
compression with resulting frictional properties that will achieve the greatest
strength. Less yarn will give insufficient compressive force, friction and
resulting strength properties. However, if too many fibres are involved, shear
will be increased and the yarn will be weaker.

Bob Wagner

Norristown, Pennsylvania

Answer: A rope is stronger than a parallel group of individual fibres because
of the 鈥渢wist鈥�. This twist ensures that during loading any eccentricity in the
load (bending as opposed to pure tension) is very quickly dissipated. The axis
of a fibre on the outer surface of a bend (and hence under higher stress) moves
to the inner surface within half a twist along the rope, and vice versa. All the
filaments therefore carry a broadly equal share of the stress.

When a similar group of parallel filaments is loaded, any eccentricity in the
load will ensure that some filaments are under a greater stress than others.
They will break first, and the load they had been carrying will be thrown onto
the remaining filaments; if any of these are also close to their breaking point
then these will break and so on, starting a tearing effect.

The effect is very real: the tensile strength of individual
cement-reinforcement glass filaments is about 3.5 gigapascals while the strength
of a strand of 204 parallel filaments is only about half of this.

Phil Purnell

Materials Research Group

Aston University

This week鈥檚 question

Stomach bugs: I once read that a famous microbiologist, perhaps Louis Pasteur
himself, demonstrated that bacteria cannot survive under strong acid conditions,
such as in the stomach. He swallowed a culture of a nasty bug and survived. If
this works all the time, why is food poisoning so common? Are the microbes which
cause it acid resistant or does infection take place before they reach the
stomach?

Paul Harthoorn

Amsterdam, The Netherlands