The suprachiasmatic nucleus really are two nuclei in the mammalian brain. Together they form the central circadian pacemaker in mammals.

The suprachiasmatic nuclei can be found just above the optic chiasm, in the hypothalamus.

The suprachiasmatic nucleus (SCN) receives input directly from the photoreceptors in the retina of the eye. Although several neural pathways and nuclei are involved in maintaining sleep-wake cycles in mammals, the SCN sets the pace by (more or less) noticing when it is daylight and when it is not - we know this because of one kick ass experiment. First, a bit of setup:

We've all had jet lag before: you fly 4+ time zones away and when you get there you feel tired, hyperactive, out of sorts, etc. for the first few days, and then you feel back-to-normal after about a week. The process of adjusting to the new time zone is called photoentrainment: that is, the modulating of your sleep cycle based upon the timing of the available daylight. And you don't even need daylight to do it. Animals and humans can be photoentrained under laboratory conditions using standard indoor lighting.

In lab conditions where all light cues are removed (i.e. continual dim light), most animals will continue to sleep and wake in a roughly 24 hour cycle, this is called " free run."

So here's the experiment: Suppose that you got yourself two Syrian Hamsters (THE sleep research animal of choice - no joke) and photoentrained them to opposing sleep-wake cycles - i.e. one of them was awake while the other slept and vice versa. (If you let these hamsters free run, you would find that they continue to stay out of phase.)

Now we perform a bit of surgery: we remove each hamster's SCN and implant it into the *other* hamster. After the surgery, if you let the animals free run, you'll see that each hamster quickly loses their original sleep-wake pattern and acquires the pattern of the *donor* hamster - they actually * swap cycles* because of the surgery. As far as we know, no other part of the brain when transplanted like this has the ability to instantly entrain this behavior, not even the pineal gland, which is important for releasing melatonin among other things.

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