I have a serious issue with table fans. If one of them is switched on within my earshot, I soon fall asleep. Some kind of hypnotic effect it has on me. So, it's best avoided when I need to stay awake and do some work. Of course, when I do need some sleep, it can provide the soporific support.
Many people seem to have the same experience, and this interesting article explains why the fan has such an effect.
If you think back to elementary-school science classes, you probably learned that white light is a combination of all the other colors of light; using a prism, we can separate it into its component colors. By analogy, "white" noise is composed of sounds of every frequency within the range of human hearing—roughly 20 to 20,000Hz (cycles per second)—with each part of the frequency spectrum equal in amplitude (volume). It's called "noise" instead of "sound" because it is random in nature. Rather than simply generating a fixed tone at 20Hz, 21Hz, 22Hz, and so on all the way up to 20,000Hz, a white noise generator creates a constantly changing mixture of tones such that all frequencies have an equal probability of being audible at any given moment.
To human ears, white noise sounds like a hiss—sounds such as a waterfall, an aerosol can, and static are all very similar to white noise. Although all frequencies are represented, we perceive white noise as being relatively high-pitched—partly because higher octaves consist of a greater range of frequencies than lower ones (giving the higher-frequency sounds proportionally more energy), and partly because our ears are more sensitive to higher-pitched sounds.
White noise is good at masking most other kinds of sound because it effectively overloads or "numbs" our auditory systems. Just as it's difficult to hold a conversation at a crowded restaurant, it's difficult for your brain to identify any one sound or voice when you're already hearing sound at every frequency. So it's not the white noise itself that promotes sleep as much as the fact that it reduces audio clutter, drowning out other sounds that may distract you and therefore keep you awake.Against this formidable effect produced by Physics and acoustics, I simply don't have a chance. I can't avoid falling asleep when the fan is switched on.