To create a Sleep Calculator application that calculates the number of hours a person has spent sleeping from the time they were born until today, we need to consider that everyone sleeps 8 hours every night, there are 30 days in each month, and there are 365 days in each year.

Here's a simple algorithm to calculate the number of hours a person has spent sleeping from the time they were born until today:

1. Ask the user to enter their birthdate.
2. Calculate the number of days between their birthdate and today's date.
3. Multiply the number of days by 8 to get the total number of hours slept.
4. Output the total number of hours slept.

To create a Javascript application based on the above algorithm, you can use the following code:

function calculateSleepHours(birthDate, today) {
  // Get the current year, month, and day.
  const currentYear = today.getFullYear();
  const currentMonth = today.getMonth();
  const currentDay = today.getDate();

  // Get the birth year, month, and day.
  const birthYear = birthDate.getFullYear();
  const birthMonth = birthDate.getMonth();
  const birthDay = birthDate.getDate();

  // Calculate the number of years that have passed.
  let years = currentYear - birthYear;
  if (currentMonth < birthMonth) {
    years--;
  } else if (currentMonth === birthMonth && currentDay < birthDay) {
    years--;
  }

  // Calculate the number of months that have passed.
  let months = currentMonth - birthMonth;
  if (months < 0) {
    months += 12;
    years--;
  }

  // Calculate the number of days that have passed.
  let days = currentDay - birthDay;
  if (days < 0) {
    days += 30;
    months--;
  }

  // Calculate the total number of hours that the person has slept.
  const totalHours = years * 365 * 8 + months * 30 * 8 + days * 8;

  return totalHours;
}

// Get the current date.
const today = new Date();

// Get the user's birth date.
const birthDate = new Date(1990, 0, 1);

// Calculate the number of hours that the user has slept.
const totalSleepHours = calculateSleepHours(birthDate, today);

// Display the number of hours to the user.
alert(`You have slept ${totalSleepHours} hours since you were born.`);

There are many existing Sleep Calculator applications available for download on app stores and online. Some of these applications use sleep cycles and sound analysis to help users determine the best time to go to bed or wake up. Others provide a simple interface that allows users to input their desired wake-up time and calculates the best time to go to bed based on sleep cycles.

Overall, creating a Sleep Calculator application can be a fun and useful project for those interested in sleep science and programming.