So you sent the email. 4:40 yesterday afternoon, two lines, a little too sharp.

You read it again this morning and winced. You knew it the second you hit send, and you sent it anyway.

You will probably blame the client. Or the week.

Maybe you just figure you are getting crankier with age.

I want to show you where that email actually came from. It started around 5am, while you were still in bed.

Your night is not one long block of sleep. It runs in cycles of roughly 90 minutes, and what happens inside each cycle changes as the night goes on.

The early cycles are heavy on deep sleep. That is the physical repair shift.

Researchers at the University of New Mexico looked at something most of us never think twice about: the water we drink every day.

What they describe is a slow build-up. Invisible toxic shards from that water collect inside the brain over time and wear down the connections that keep your memory sharp.

The part that matters most is the pace. According to the findings, this speeds up memory loss by 50% every 8 years.

It happens quietly, a glass at a time, which is why it is worth understanding early.

Here is what is erasing memory day to day, and how to stop it:

>> See what’s damaging your brain every day

The later cycles tilt toward REM. In the last few hours before your alarm, REM periods run their longest of the whole night.

REM Sleep

The stage where your brain runs almost as hot as when you are awake, your body is locked still, and noradrenaline, your brain's stress chemical, drops to its lowest point of the 24 hours.

That last part is what floored me.

REM is the one time your brain can replay the day's hard moments without the stress chemistry attached. The argument, the bad number, the guy who undercut your bid.

Sleep researchers at UC Berkeley call it overnight therapy. You go to bed with the sting and wake up with the memory, minus most of the charge.

Now watch what the alarm does.

Set it for 5 instead of 6:30 and you did not trim your night evenly. You cut off the end, and the end is where REM is stacked.

You lost about a quarter of your sleep. You lost a much bigger slice of your REM.

What follows is not subtle. A 2007 study from UC Berkeley and Harvard put sleep-deprived people in a brain scanner and showed them upsetting images.

Their amygdala, the brain's alarm bell, fired about 60 percent harder than in people who had slept. And its link to the prefrontal cortex, the part that says "easy, do not send that," had gone weak.

Bigger alarm. Weaker brakes.

That was a full night with no sleep, the extreme version. But the direction is what matters: shave the end off enough nights and you walk around with a hair trigger and soft brakes.

So yesterday's sting never got filed. You carried it into today, and it came out at 4:40 in a two-line email.

That is not your personality. That is a skipped shift.

And it runs further than one email. Negotiating, managing people, holding your tongue with your partner when the numbers are bad: all of it runs through the same brakes.

A man with soft brakes leaves money on the table and calls it bad luck.

The alarm is not the only thing stealing those last cycles. Booze is another: it pushes REM down early in the night, and the back half falls apart around 3am.

Then there is waking at 4 and staring at the ceiling. That is your REM window, and you are spending it wide awake, answering your inbox in your head.

Some of that is chemistry. Staying asleep leans on the calming side of your nervous system, the GABA system, and magnesium is part of how that system settles things down.

Roughly half of American adults do not get enough magnesium from food. A good magnesium or sleep-support formula aimed at the back half of the night is worth a look.

And if your wife says you snore hard or stop breathing, take that to a doctor. Breathing problems in sleep often get worse during REM, and they break it into pieces.

Tonight, leave the alarm where it is. Move your bedtime 45 minutes earlier and guard the end of the night like billable hours.

Now think about what one bad email can cost. A client walks, a deal goes cold, or a good employee quits because you bit his head off on a Thursday.

Ninety minutes of REM costs you a bedtime. Do the math, then go to bed.