I Built a Free Sleep Calculator in Pure HTML/CSS/JS — No Frameworks, No Libraries, 98 Desktop Speed

Why I Built This:

I couldn’t find a sleep calculator that was:

• Fast on mobile
• Free without ads
• Actually explained the science
• Didn’t require signup

So I built one from scratch in pure HTML/CSS/JS.

What is Sleep Calculator REM?

sleepcalculatorrem.com is a free tool that
calculates your optimal wake-up and bedtime
windows based on 90-minute REM sleep cycles.

Most people wake up groggy not because they
slept less — but because they woke up in the
MIDDLE of a sleep cycle.

This tool solves that.

Features

3 Calculation Modes

• Wake Up — enter bedtime → get best wake times
• Bedtime — enter wake time → get best bedtimes
• Sleep Now — calculates from current time instantly

Special Features

• 🧠 Visual REM cycle diagram
• 👶 Age-based sleep recommendations
• 📊 Sleep debt calculator (weekly tracker)
• ⚙️ Custom cycle length (85–110 min)
• ⏱️ Custom fall-asleep time setting
• 🌙 Dark mode by default
• 📋 One-tap copy alarm time button

Tech Stack