that nicotine may affect sleep in other, more indirect ways too. Smoking is believed to affect blood sugar levels, which can make you irritable, and smokers are also statistically more likely to be coffee-drinkers, the combined effect of nicotine and caffeine having a disastrous long-term impact on sleep duration and quality.
Environment
Studies have shown that the sleeping environment can have a great impact on sleeping patterns. The degree of noise, vibration, light, humidity, or sharing a room with a partner who snores – all can have a significant part to play.
must know
Light and the retina
The retina consists of receptors that are sensitive to light. One group known as the ganglion cells are particularly sensitive to light, and in particular to blue light (such as that of the sky). These cells make a direct connection to the biological clock (see page 14) which controls the time that we are likely to sleep. About 20 per cent of light gets through the eyelids when the eyes are shut, which means that we can be affected by light and the time that we sleep even when we are in bed with our eyes shut!
Light
Light can have a profound effect upon our wellbeing, both consciously and unconsciously. The conscious effects of light have been known for many years. Numerous studies have shown that subjects exposed to bright light experience significant improvements in mood and a decrease in feelings of tiredness. Indeed, conditions such as Seasonal Affective Disorder (SAD – see page 17) are even treated by exposure to bright light. In recent years, scientists have learned that it is blue light (or natural skylight) that has the greatest effect on our sleeping patterns, because of the unique and very special influence it has on our internal bodyclock – the mechanism that determines what time we go to sleep at night and get up in the morning. (See The brain’s metronome, see page 16).
Any disruption to the bodyclock can lead us believe that it is daytime rather than night, and to react accordingly. Blue light, it is believed, also inhibits the night-time secretion of melatonin, the hormone that signals the onset of darkness (see page 16), thus preventing or disrupting sleep. (When it is dark, melatonin secretion rises and peaks at the darkest time and then goes down until dawn, when the biological clock is reset to wake.) The effects of blue light are unconscious.
There are also conscious effects of perceived light: the brighter the light in your bedroom, the more alert and less sleepy you will feel. Taking lux as a measurement of light intensity (see the table below), normal indoor lighting measures around 200 ‘lux’, while a cloudy day is around 10,000 ‘lux’ – significantly lighter, which makes it crucial to block it out as much as possible when you go to sleep. If you cannot control the amount of light you are exposed to and don’t want to hide your eyes with a pillow, consider using an eye mask.
did you know?
Blind or visually impaired people have no internal bodyclocks because their retinas are so damaged that light cannot enter the brain. They will therefore have no natural sense of when to get up and when to go to bed.
In recent years, street and industrial lighting has increased dramatically, which has had a great impact on normal daylight hours and in some people can have the effect of disturbing the timing of their internal clocks. Again, an eye mask, thick curtains, or curtains with extra lining may help.
The relative intensity of different forms of light
| Illuminance | Example |
|---|---|
| 0.00005 lux | Starlight |
| 1 lux | Moonlight |
| 400 lux | A brightly lit office |
| 400 lux | Sunrise or sunset on a clear day. |
| 1000 lux | Typical TV studio lighting |
| 32,000 lux | Sunlight on an average day (min.) |
| 100,000 lux | Sunlight on an average day (max.) |
Note: Lux is a measure of the intensity of light.
must know
Decibels
Noise is measured in decibels (dB). A 3dB change is detectable but because of the scale used with decibels an increase of 3 means that the noise level has actually doubled.
There is some evidence to suggest that using coloured lighting may affect the ‘colour temperature’ of a room. Colours can be pleasing to some people and irritating for others so it is important not to be too assertive as to which are best. However, in general the warmer colours of sunset tend to be associated with deep sleep while the cooler colours of dawn are associated with darkness.
Humidity
One of the most common breathing-related sleeping disorders is asthma, which is often caused by allergens found in the faeces of house dust mites. The relative humidity of your surroundings will be the key factor that influences the prevalence of these mites, which can live in conditions where there is no liquid to drink as they can extract sufficient water from the environment if the humidity is high enough. Surveys in temperate climates show that mite prevalence varies according to seasonal fluctuations in indoor humidity. Mites are absent or rare in homes in dry climates unless use of evaporative coolers adds the moisture to the air that is necessary for their survival. Maintaining average daily indoor humidity below 50 per cent will prevent mite population growth and subsequent sleep-disrupting allergens. Even if humidity is lowered, it can take several months for all the allergens to disappear, however.
Noise
People can be awoken by neutral sounds louder than 45 decibels (dB), the equivalent to someone talking quietly (a lawnmower is around 105dB and the threshold for injury to the ear is around 140dB). However, sounds as low as 20dB (below the hearing threshold) can prevent you from falling asleep.
Conversely, continuous background sounds can be soporific and it seems that sounds even below 20dB can induce sleepiness – a clear danger for lorry and train drivers. Irregular high-frequency noise can have the opposite effect and be intrusive.
Noise table
| Type of noise | Decibels (dB) |
|---|---|
| Faintest, audible sounds | 0.2-20 |
| TV sound studio, quiet library | 20-30 |
| Quiet office | 40-50 |
| Conversation | 50-60 |
| Primary school classroom, loud radio | 60-70 |
| Power drill | 85-90 |
| Road drill | 100-110 |
| Chainsaw | 110-115 |