More and more people are taking their iPads to bed with them to surf the web, check facebook or send e-mails before switching off the light. But researchers are warning that the blue light their screens give off can stop users getting a good night's sleep. That is because this type of light is similar to daylight, convincing the brain that it is still daytime. Blue light prevents the production of a brain chemical called melatonin, which helps us fall asleep. By contrast, light which is more orange or red does not prevent melatonin production, perhaps because our brains recognize it as a cue that the day ends. Scientists have known for years that staring at screens late in the evening can affect sleep ---- they are television screens, computer screens or mobile phone screens. However, because mobiles and iPads are by nature portable ---- not to say addictive ---- more people are taking them into the bedroom. Users also tend to hold them much closer to their eyes than a computer or television screen. Researchers are warning that looking at iPad displays for more than two hours leads to lower natural melatonin levels as the devices give off blue light. They say, "Turning off the devices at night is the final solution. But if you have to use these devices at night, dim the display to cut down irritation of the eyes and limit the time you spend on them before bed." They wrote in the journal Applied Ergonomics  that iPad makers should adjust the spectral power distribution  of the devices so that they affect the sleep patterns of users less. It is not just a good night's sleep that could be harmed by too much late night screen time. Researchers know that long-term harm to sleep patterns can lead to an increased risk of obesity and even cancers. Blue light prevents iPad users from sleeping well because   _  .
it is the same as daylight which keeps people awake it makes the brain more active it is brighter than light that is more orange or red it can help decrease the production of melatonin