Why unfamiliar voices disrupt sleep

When you try to sleep for the first time in a new place, you may find it hard to settle. While tossing and turning might leave you feeling knackered the next morning, your brain thinks its constant surveillance is doing you a favour. But how does your brain tell a safe noise from a real bump in the night? Julia Ravey interviews Mohamed Ameen from the University of Salzburg…

Julia - Every night we lay our heads down and slip into a state of semi unconsciousness. Certain noises you are used to don't disturb your slumber. Even familiar voices (Julia, Julia) like Naked Scientist, Harry Lewis, can be ignored. But if you heard an unfamiliar voice (Julia!), your eyes would most likely ping open, and you would be on hyper alert. But how does our brain decide what we should wake up to and what we should ignore? I spoke to Mohamed Ameen from the University of Salzburg who investigated just that...

Mohamed - When you're asleep, you're not completely unconscious. There is at least a very low level of monitoring the environment that is going on, and that was already evident years ago when scientists showed that sleepers respond selectively or preferentially to their own name. This means that we can at least still have a low level of processing that enables us, even during the deepest stages of sleep, to tell the difference between different stimuli.

Julia - There must be a fine balance of the brain wanting to keep us asleep, but also not wanting to miss out on any potentially threatening situations. So, how does it decide what sounds are normal and what could potentially be something that we need to be aware of?

Mohamed - When a certain stimulus is presented, the brain responds in a way that facilitates a very low level of processing that then tells you, 'Okay, this stimulus is normal. This is something I expect in this environment', or 'this is something that I don't expect'. And then, for a very, very short time, the brain goes into more wake-like activity and then decides whether to disrupt, sleep or continue. And this is like, 'okay, it's fine'

Julia - What stimuli did you use in the study?

Mohamed - We used the subjects first names and other different names. But the difference is that these names were spoken by either a familiar voice or an unfamiliar voice. A familiar voice is a voice that you're used to you - your parents, your partner - and an unfamiliar voice is a voice of someone you've never heard before. We couldn't find a very strong effect of the name, and we think that this is masked by the much stronger effect of the voice, because this is probably a much simpler stimulus to process. So, at this very level of voices, the brain can already tell the difference, even when you're in deep sleep.

Julia - Is that why, when I stay in a hotel, if someone walks past the door and they speak, I wake up. But if I was at home and someone walked past my door and spoke, because it'll be like my family member, I don't necessarily wake up. Is that potentially why that happens?

Mohamed - It could be, yes. The idea is that you are familiar with the surrounding environment. So, for example, when people first sleep next to a train station in the beginning, you always wake up when the train passes. And then, after time, you realise that this is a familiar stimulus that imposes no danger, and there's no need to disrupt your sleep.

Julia - I used to live on an ambulance route in London and, when I first moved there, I was woken up all through the night because ambulances would be going past all the time. Within a few weeks, I didn't wake up to those sounds at all. With your experiment, did you see anything similar with these unfamiliar voices?

Mohamed - We saw something that is within the night. We realised that the brain can even learn something about the stimuli that are presented. When the brain realised that this is an unfamiliar voice, it elicits certain brainwaves and, these brainwaves, we saw a lot of them in the first half of the night. In the second half of the night, as the stimuli are repeatedly presented, these responses decrease gradually, specifically to the unfamiliar voice. This suggests that the brain is learning, 'okay, this has happened before but imposed no danger' and it learns, it predicts something, it gathers information and compares its prediction to what it actually received or heard.

Julia - Sort of like: 'that sound is nothing to worry about. You don't need to wake up from your eight hours sleep, no point disturbing that really precious sleep that we get.'