Environmental light . Bright light tends to increase alertness while dim light leads to drowsiness. Cool, dry air, especially on your face, helps keep you alert, while heat and humidity make you drowsy. The sound of rolling waves on the beach or the hum of white noise from machines can lull you to sleep. Irregular or variable sounds, such as a radio, conversation or a honking horn, stimulate alertness.
Studies have found that some smells like peppermint, make people more alert.
Others, like lavender, meanwhile, have a sedative effect. An interesting truth about alertness is that it is sometimes incompatible with comfort. In fact, to be fully alert, we may need to be a little uncomfortable at times. Too often people performing important tasks try to make themselves comfortable. However, when many of the nine switches are turned off, alertness sags and dangerous events such as microsleeps and automatic behavior occur.