When run, will return in a spreadsheet the number of emails in your inbox, as well as the number of emails received the previous day. Run it with a daily trigger at 1am.

gmailStats.gs

function GmailEmails() {
  var InboxThread = GmailApp.getInboxThreads().length;
  var today = new Date();
  var year = today.getYear();
  var month = today.getMonth() + 1;
  var day = today.getDate();
  var day1 = day - 1;
  var day2 = day;
    
  var yesterdayThreads = GmailApp.search('-from:me after:' +year +'/'+month +'/'+day1 + ' before:' +year +'/'+month +'/'+day2);
  var threadCount = yesterdayThreads.length;
  
  var gmailSheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Gmail");
  gmailSheet.appendRow([new Date(),InboxThread,threadCount]);
}