Renewing certificates

I'm setting up certbot/letsencrypt to provide a certificate for dovecot and sendmail. Is it necessary to restart dovecot to load the new certificate, as shown in most examples I find in blogs? That seems rude to established connections. When does dovecot read the cert and key files? Once at startup or each time a connection requests SSL? Is there a preferred locking protocol when changing the two files to keep dovecot from reading one while the other is being replaced and getting a mismatched pair?